Hi Loiroi,
If your mattress feels different from the one in the store then outside of any differences in the materials and components themselves which would be a manufacturing error and would be unlikely …there are several possible reasons that could account for the difference in your experience between the showroom and sleeping on the mattress in your bedroom.
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The mattress in the store may have been broken in more than your mattress. There will be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress (higher density materials can take longer) and it can be surprising to many people how much their sleeping experience can change over the course of the first month or so and I would tend to sleep on a new mattress for a minimum of 30 days or so if possible before deciding on whether the mattress will be suitable for your needs and preferences in terms of PPP over the longer term.
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The support systems underneath the mattresses could be different. The most suitable support system for a foam mattress should have a firm, flat, and evenly supportive support surface underneath it that has minimal to no flex under the mattress and for larger sizes with at least one center support beam that has good support to the floor to prevent any sagging in the middle of the mattress. It should provide similar support to having your mattress on the floor. If the support system under your mattress has more or less flex than the support system under the mattress in the store or it allows your mattress to sag into any gaps in the support surface then this can change the feel and performance of your mattress.
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Temperature and humidity can both affect the feel and firmness of memory foam so it’s possible that any significant temperature differences between your bedroom and the showroom could have some effect on the firmness of your mattress.
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The mattress pad or mattress protector, or even the sheets you are using can also have a significant effect on the feel and performance of your mattress and as Daniel mentioned this could be the most likely reason for the difference. A thicker mattress pad or to a lesser degree your mattress protector or your sheets can affect the ability of the top foam layers to contour to the shape of your body which can affect the feel, firmness, pressure relief, and secondary support (the type of support that “fills in” the gaps in your sleeping profile) of your mattress and it can also reduce the amount of body heat that reaches the memory foam which can firm up the memory foam underneath it or at least slow down the rate that it gets softer as you sleep on it.
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The pillow you are using at home could be different from the pillow that you used when you tested the mattress and this can also make a difference in how a mattress feels as well (particularly in the upper body).
Outside of any differences in the mattress or “sleeping system” itself … there are other variables that could account for the difference as well.
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Human memory for softness, firmness, and “feel” is very short term, subjective, and relative to more recent experience and is often unreliable. A mattress that feels different to what someone “remembers” a mattress feels like may end up being more similar than they remember (or vice versa) if they were to compare them side by side in “real time”. Most people have had the experience of testing mattresses say in the morning and then testing mattresses somewhere else and then going back to test the first mattresses again later in the day and finding they feel different from what they remember because their frame of reference has changed with the other mattresses they have tested.
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It could also be related to “how” you tested the mattress in the store. With a local purchase and for the majority of people … careful testing using the testing guidelines in the tutorial rather than just testing for the more subjective “comfort” or “showroom feel” of a mattress and some good guidance from a knowledgeable and experienced retailer or manufacturer will usually result in a mattress choice that is well inside a suitable comfort/support range and will generally be “close enough” so that if any fine tuning is necessary it would be relatively minor and involve different mattress pads, sheets, mattress protectors, or perhaps even a topper if a mattress is too firm.
Careful and more objective testing is different from the more limited or more “subjective” testing that most people do when they choose a mattress based on “showroom feel” which generally isn’t a reliable way to predict how well you will sleep on a mattress or how it will “feel” when you sleep on it at home and can have a lower chance of success than random chance alone … see this study.
Many people don’t spend enough time on a mattress in all their sleeping positions to really be able to predict what it will feel like or how they will actually sleep on it once they have fully relaxed (like they would be when they are going to sleep at night) which can lead to an impression that a mattress that you purchase is different from the one you tested in the store or result in a sleeping experience that is different from your experience in the store. There is more about more “subjective” testing vs more careful and “objective” testing that has a much better chance of predicting how you will sleep on a mattress in post #4 here and post #10 here).
Having said all that … there are certainly some people who are exceptions and are much closer to the “princess and the pea” end of the sensitivity scale than they are to the “I can sleep on anything” end of the scale and because of this or because they may have a more challenging physiology or health conditions that can make choosing a suitable mattress more difficult or because of a history of choosing a mattress that isn’t the best match for them they may not be as confident that even careful testing will be able to predict whether they will sleep well on a mattress. In this case then the options you have after a purchase would become a more important part of your personal value equation and purchase decision so if a mattress you choose doesn’t turn out as well as you hoped for you have the ability to make changes to your mattress either by changing individual layers or exchanging the mattress or even having the option to return the mattress for a refund after a purchase.
I’m not sure why you are using a mattress pad but it may be an idea to replace it with a thinner and more flexible mattress protector that has the least possible effect on the foam layers in your mattress.
There is more about the pros and cons of different types of mattress protectors and some examples of each of them in post #89 here.
Phoenix