Mattresses are sold with firmness ratings that appear comparable across brands. There is no shared scale behind them, and the same word means different things to different makers.
The scale is internal to each brand
A manufacturer decides what its own medium means and positions its range around that decision, usually so the range spreads sensibly across the labels it uses.
Nothing requires that decision to align with anyone else's, so one brand's medium can sit where another brand's firm does without either of them being wrong.
Numeric scales suggest more precision than exists, since the number describes a position within one range rather than measuring any physical property of the product.
Why body weight changes the result
Firmness as experienced is a relationship between sleeper and surface, because how far the materials compress depends entirely on the load being applied to them.
The same mattress therefore reads softer to a heavier person and firmer to a lighter one, and the gap can span more than one label on the brand's own scale.
Sleeping position adds to this, since side sleeping concentrates load over a smaller area than back sleeping and pushes further into the comfort layers.
What is actually being described
Firmness refers to the initial feel of the surface, while support refers to whether the structure keeps the spine aligned under load. The two are independent of each other.
A soft-feeling mattress can be well supported if the layers beneath the comfort surface are firm enough, and a firm one can support poorly if the core is inadequate.
Descriptions rarely separate the two clearly, which is why a mattress can match a preferred firmness rating and still be unsuitable for the person buying it.
Why the feel changes after delivery
Compressed and rolled mattresses need time to recover their full loft, and the surface reads firmer than intended until the materials have fully expanded again.
Foams also soften during an initial period of use as the structure settles under repeated loading, which is normal and quite distinct from premature failure.
Trial periods commonly require a minimum number of nights before a return is accepted, and that requirement exists precisely because of these two effects.
How to compare across brands
Construction detail is more informative than the label, so the useful facts are layer thicknesses, foam densities, spring counts and whether the springs are individually pocketed.
Density in particular predicts how long the comfort layers last, because low-density foam loses support faster under repeated compression regardless of how it feels when new.
Trial length and return terms carry real weight, since firmness cannot be assessed reliably in a showroom and is only established over consecutive nights at home.