The same thing happened to me when I got my new bed in the summer. It definitely was firmer than the one in the store. So much so that I checked out everything on the label carefully and went back to the store to compare with the bed I tried there. Surprise - the bed in the store didn't have the label naming but and the salesperson insisted it was the same.
I can only assume it's true that all the people testing by bouncing, rolling and plopping down on the floor model softened it up. I got a soft topper for mine and it's perfect now. Actually, I'm glad I have it since I don't think the firm mattress,with less foam on top, will indent as quickly as a plush, soft one would.