The shop floor model display you tested will have been used and tested by various customers several times before you tested it.
Your new mattress will soften over a period of time, particularly where you lie, showing that your new mattress is working with the pressure you exert on it.
Please note it can take several weeks for the mattress to fully settle.
You will need to rotate or turn & rotate you mattress to allow the fillings to settle correctly. Not turning your mattress will also invalidate your mattress as it is part of the product care.
More information about rotating your mattress can be found here.