Hi paisley,
Even the firmest springs with an insulator over them to even out the feel of the individual coils would be softer than a floor because they compress with pressure while a floor doesn’t. The quilting layer also has two 3/4" layers not just one so the thickness is 1.5" in total. This would generally be soft foam but the quilting pattern and the way that a tighter quilting pattern compresses the foam can make a quilting layer firmer than the same foam would be in an unquilted layer.
I certainly agree that there is “something going on” and it’s not your imagination but the challenge is to figure out what and this can take some trial and error and some detective work along with some mistakes. There are many things that can produce the same “symptoms” on a mattress and sometimes it’s easy to make changes in the wrong direction and make a symptom worse or replace one symptom with a different one. Unfortunately they don’t list the specifics of the layering in your mattress or the ILD so there is no way to use this to make better guesses. Individual perception is also so subjective and depends so much on how different body types and the variations in each person’s sleeping positions interact with the same mattress that any advice based on “theory at a distance” is educated guesswork or speculation at best. This is the reason why a return policy is such an important part of the “value” of a purchase when you are in an experimental mode because there is no formula that can predict which mattress/topper combination will work best for you with any certainty … especially when there are so many unknowns.
The feel of a mattress depends more on the top layers of the mattress (which are more connected to pressure relief) than the deeper layers of a mattress and the lighter you are the less you will “feel” the effect of the deeper layers. There is a point where you could put almost anything under a thick enough foam layer on top and you wouldn’t be able to feel what was underneath it. While it’s true that lighter body types will compress the layers less … it also depends on your body shape and your weight distribution as much as your weight and height. Some people for example that are side sleepers and have wider and heavier hips may have more weight in the pelvic area and they may “feel” more of the deeper layers there than they do under their shoulders which are lighter. Weight distribution and body shape along with sleeping positions (and there are dozens of sleeping position variations) are just as important as your overall height and weight.
Pressure relief and “comfort” is what you tend to feel when you first lie on a mattress and this comes mostly from the upper layers.
Support and alignment is what you tend to feel when you wake up in the morning either with or without back pain or discomfort. This is where the deeper layers can have a more significant effect.
Durability is all about how you feel on a mattress in a year or more down the road when lower quality materials have softened more rapidly.
Since your mattress is not new it’s also possible that there are soft spots in the mattress that are allowing the heavier parts of your body to sink down more than they should be and if this is the case then a topper will only be a temporary of partial “fix”. A topper does best with an even mattress surface that has no sagging or soft spots. You can read more about this in post #4 here. This is one of the most common problems with mattresses that use lower quality/density materials in the upper layers of the mattress.
You aren’t “missing” anything … it’s just that you are sensitive to whatever is the underlying cause of the symptoms you are experiencing. Some people “feel” things that others don’t. This is what I call the “princess and the pea” vs the “I can sleep on anything” scale. All of us are somewhere inside this scale. Some people with the same body type as you could put a 3" topper on top with the same ILD that you have or say a 19 ILD and it would be perfect. Others would think it was the worst combination that they had ever slept on. This is why “theory at a distance” or formulas are only based based on averages and averages don’t work for everyone.
Again … there is no way to know this. The truth is that a 3" topper in the range of 19 to 24 ILD would work for a large number of people that are in similar circumstances as you and when you aren’t inside the “averages” then it’s really a matter of trial and error.
The one thing I would confirm is that the surface of your mattress has no soft spots and is completely even because I suspect that after 5 years the odds are high that it does and this could also be part of the problem.
Phoenix