Hi 2ndNature,
It’s not unusual at all for a new mattress to feel firmer than a floor model and a mattress can take a few weeks to go through it’s initial softening and break in period. There are also other factors that can make a difference in the feel of a memory foam mattress including the temperature or humidity of your bedroom, the mattress protector you are using, or your sheets and bedding. Post #2 here has more information that may be helpful. It would be rare that there was a significant difference on the manufacturing side of things.
I would go through the “list” to eliminate the other factors that could be involved and give it 30 days or so.
The good news is that you were wise enough to choose a mattress that was a little firmer because it’s much easier to “fine tune” a mattress that is too firm with a topper than it is to firm up a mattress that is too soft (which would generally involve removing layers rather than adding to them).
Phoenix