Hi a2dox,
There’s no way for me to know this of course but designing your own mattress can be somewhat tricky because there are many seemingly small things that can affect how it feels and performs that can sometimes be surprising.
The difference in the support core would probably have “some effect” but with 7.5" of foam on top of it the chances of it being a major difference are much smaller.
Glued layers vs unglued layers can also make some difference and any difference in the cover can also affect the feel and performance of a mattress as well (especially with memory foam which is temperature sensitive and can be affected by how much of your body heat reaches it).
Your platform/bedframe would have very little give so it’s also less likely that it would be affecting how your mattress feels compared to the solid surface you tested it on.
Some other possibilities are that your mattress is very new and there will also be a break in and adjustment period with any new mattress and it’s also possible that any mattress protector or mattress pad you are using may be having an effect on your mattress as well (see post #2 here).
It’s also possible that it’s connected to how you tested the layering in the store and the more subjective differences between what you felt when you tested the mattress and what you feel when you sleep on it could be contributing to the differences you feel as well. Our memory of firmness and softness is not always accurate and is also fairly subjective and relative. If there is some time between when you test a mattress and then test or sleep on it later then the time and your experiences in between can alter your perceptions of how soft or firm it is as well. It’s not uncommon for example to test mattresses early in the day and then go back and test the same mattresses later in the day or the next day after you have tested other mattresses and discover that the mattresses that you tested feel different than you remember they did because your frame of reference has changed with time and as you tested other mattresses.
Finally it’s also possible that your layers are somehow different from what you tested but of course there would be no way for me to know this.
The “best” suggestion for now though outside of the suggestions regarding your sheets and bedding and talking to Paul at Rainbow Bedding to see if they have any suggestions as well would probably be to “do nothing” to see how the normal break in and adjustment period for any new mattress affects your experience.
Phoenix