It won’t. You need to manage expectations for customers and talk to them about the initial stiffness that any new mattress will have, from the ticking to the flame retardant layer to the stitching to the foams, and how all of this will soften up a bit in the first few months.
You can even take three of the same mattress from your back room and find slight differences between them due to the slight variances in foam hardness and assembly. And, while customers who claim they can tell subtle differences from a showroom model to their mattress at home are almost always overestimating their tactile skills (and forgetting to take into account the impact of a mattress pad and fitted sheet), that still doesn’t mean you should avoid explaining to people that a new mattress will feel slightly firmer than the floor model, as it avoids so many potential issues (I print this on the mattress care guide I provide all my clients).
When your customers are testing out mattresses in the showroom, if they try something and say that it is on the cusp of their softness preference, move them back one level of surface hardness and show them something that is a bit less plush, as all foams, even latex, will soften over time, and you want to think longitudinally for your customers, realizing that this mattress will feel softer in three to four years than it does in the showroom, perhaps becoming softer than your customer might desire. And if your customers still demand a mattress that feels exactly like what they tried in the store, sell them the floor model.