Obviously every mattress can’t be exactly the same and can’t be exactly to spec as far as size and weight. Given size and density of layers you can calculate how much a memory foam mattress should weigh. I’m wondering what is normal and what isn’t as far as variation on size and weight of the product you receive.
For a King for instance that is supposed to be 75-79, would it be normal to get one that is actually 74-78? Or one that should weigh 140lbs to weigh 135? What is normal and what is defect?
Foam specs are never exact and when a mattress manufacturer orders foam from a foam pourer they can specify a spec tolerance that they wish to use (tighter tolerances are more costly) but it would be fairly normal to see a spec tolerance of about 10%. This means that a 2 lb density polyfoam for example could be 1.8 lbs density or a 4 lb density memory foam could be 3.6 lbs density. Some materials such as Dunlop latex can also vary depending on whether a layer was cut from the top or the bottom of the original core and all latex will vary in density based on firmness (unlike polyfoam or memory foam).
There can also be some dimensional variation which can affect weight because a mattress that is say 60" x 80" may have internal foam layers that are an inch or so less to allow for the thickness of the side panels of the mattress.
Overall though it would be reasonable to see a variance in the range of about 10% or so and the examples you gave are very close to “exact” and would be well within a normal variance.
If your mattress contains polyfoam or memory foam then the actual weight of the mattress (the listed shipping weight isn’t always accurate) should be reasonably close to a calculation based on the listed densities, size, and thickness.
If your mattress contains latex then the weight would vary depending on the firmness of the latex (softer latex is a lower density than firmer latex).