Hi awoods,
ILD’s are never exact with latex (or any material for that matter) and they are always in a range. There is more about this in post #6 here. Many retailers or manufacturers will specify a single number which is usually just the “target” ILD or the midpoint of the range. ILD variances in the range of about 2 or 3 ILD or sometimes even more would be undetectable for most people.
I would tend to avoid using comfort specs to choose a mattress since most consumers don’t have the knowledge or experience to really know how to translate them into meaningful terms and with careful testing your body will tell you more about whether a mattress is a good match for you in terms of PPP than comfort specs. ILD is also only one of many factors that can affect the softness of a material or a mattress and by itself can be as misleading as helpful (see post #4 here).
Comfort is a very subjective term and involves a combination of pressure relief, alignment, the surface feel of a mattress, the overall feel of a mattress, and other aspects that affect how “comfortable” someone may be sleeping on a mattress such as motion isolation, motion restriction, and temperature regulation so it really doesn’t have any specific meaning.
Having said that … with polyfoam and memory foam density is mostly related to the quality/durability of a material and with these two types of foam any density can be manufactured in a wide range of firmness levels (and with other properties besides firmness). Density does have some relationship to compression modulus though (higher density polyfoams and memory foams tend to have a higher compression modulus) and since compression modulus is also one of the factors that can affect the softness of a foam material it can have “some effect” on how soft or firm a foam feels when you sink into it more deeply.
With latex on the other hand … density is a comfort spec not a quality spec and is directly related to ILD so firmer latex will have a higher density than softer latex of the same type and blend. I would also keep in mind that because ILD is only one factor in the firmness of latex (or any foam material) and because ILD is sometimes tested using different methods or thicknesses of test material that different types and blends of latex that have the same ILD rating may not be the same softness/firmness, different types and blends of latex that have the same density almost certainly won’t be the same ILD, and even different types or blends of latex that are tested in exactly the same way and have the same ILD may not feel like they are the same softness because of the differences in some of the other specs involved with how firm or soft a foam feels to someone.
I would also keep in mind that support isn’t only about firmness or softness because the goal of a mattress is good alignment in all your sleeping positions and a mattress that is too firm can be just as non supportive as a mattress that is too soft depending on which part of the body needs to be supported. There is more about primary and secondary support and their relationship to pressure relief in post #2 here.
If you begin to focus on specs (other that the “quality specs” that you need to know that you can’t feel when you test a mattress) then you may quickly reach a point of “information overwhelm” and “paralysis by analysis” and still be no further ahead in being able to decide which mattress is the best match for you in terms of PPP because your body will be much more effective at knowing this than your mind.
Phoenix