Hi nitz369,
Yes … there is an initial break in period any new mattress where the foams will lose any of their “false firmness”, any fiber materials will compress and settle to some degree, and the cover will lose any of its initial stiffness. You will also go through an adjustment period over the course of the first few weeks as well as your body gets used to the feel of a new mattress that is different from what it’s used to. While it may not be enough to turn a mattress that feels firm to you into a mattress that feels soft … the first few weeks that you sleep on a mattress can certainly make a difference in how it feels to you. There is more about the initial break in and adjustment period in post #3 here and in post #2 here.
If you are using a thicker mattress pad instead of a thinner mattress protector then in some cases it can also reduce the ability of your mattress to contour to the shape of your body as well and can make it feel firmer … even if the mattress pad itself feels softer.
I would also keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and very subjective and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here). In other words you will always need to test a mattress or sleep on it in person for you to assess how firm or soft it feels to you regardless of whether it feels firm or soft to someone else or how anyone else may “rate” it.
The good news is that you had the foresight to choose an online mattress with a good return policy that you can test in your bedroom instead of a showroom so if once your mattress has had a chance to break in and you have had a reasonable chance to adjust to it if for any reason you aren’t happy with it then you can return it for a minimal return cost (they only charge the cost of the initial shipping for a return/refund).
The better options and possibilities I’m aware of in the Las Vegas area (subject to the quality/value guidelines here) are listed in post #2 here.
I would also keep in mind that latex and memory foam are completely different materials that have a very different “feel” so I certainly wouldn’t use her experience on a Tempurpedic memory foam mattress as an indication of what latex will feel like. There is more about the differences between them in post #2 here.
Phoenix