Hi Automaton,
There are a few possibilities that could be happening.
The first is that your mattress testing didn’t predict your actual sleeping experience as well as it could have. Depending on the type of testing you do … testing a mattress in a showroom doesn’t always predict your actual sleeping experience. There are some testing guidelines in the tutorial post here which can be helpful.
It’s also likely that it could be connected to the normal break in or adjustment period that is part of any new mattress purchase. All mattresses will be a little firmer when they are new than they will after they are broken in and the layers lose any of their “false firmness”, the materials settle to some degree, and the cover loses some of its initial stiffness. The mattresses on the showroom floor are usually a better reflection of how a mattress will feel once they’ve broken in than a new mattress.
There are also some suggestions in post #2 here that may be helpful if your mattress is firmer than you hoped for but the “best” suggestion for the first few weeks would normally be to do nothing (if possible) so that you can assess the mattress after a few weeks.
If you do decide to do an exchange then I would make sure that you do some very careful and objective testing on the other mattresses that are available to you because a mattress that is too firm can easily be “fixed” by adding a topper but if you go to the other extreme and choose a mattress that is too soft then it’s much more difficult to “fix” because you would usually need to remove and replace the materials inside the mattress that are too soft rather than adding a topper on top of them (see post #4 here).
I would also be very cautious about using reviews as a way to decide on a mattress (see post #13 here).
Phoenix