Tempur pro Luxe smart cool - so confusing!!!

Can anyone pls explain why the medium firm might feel softer than the medium? - I’m hoping it wasn’t down to the showroom and that I’ve made the right decision to go with the medium firm, I’m wanting a mattress that doesn’t sink too much, but isn’t too firm, a tempur rep said it’s correct, that it does feel softer, bit couldn’t explain why - why on earth would this be?!! I’m 5”4 and 8.5st and after reading comments, I’m half expecting a brick to arrive 🫠