Hi kvinPA,
That’s mainly because “softness” and “firmness” have several different meanings and can be quite misleading. The support layers of a mattress are generally too firm to sleep comfortably on but are there to keep you in alignment and to “stop” the heavier parts from sinking in too much. This requires much firmer foam than comfort layers which are designed to form a cradle and relieve pressure. Typically … foam support layers start at about 28 ILD and go up from there. 28 ILD is at the softest end of the support layers and would be considered “soft” in terms of support (the ability to hold up your heavier parts).
On the other hand … comfort layers are for pressure relief and they can vary from about 10 ILD (supersoft quilting foam and many memory foams) up to about 24 ILD. All of these are in the soft range. In some cases even 28 - 32 ILD may be used but these would be considered relatively firm in terms of pressure relief and would be more “medium”. They would be more suitable for heavier people (firmer foams feel softer for heavier people) or for back or stomach sleepers who don’t need to sink in as much).
The thickness and softness of the upper layers will also play a big role in how firm a mattress feels. For example … if you have a 2" layer of very soft foam and put this over a 40 ILD base layer … you would “go through” the upper layer and feel much more of the firmer layer below it. 2.5 lb memory foam is so soft that much of what you are feeling is the relative firmness (in pressure relief terms) of the base layer below it. 3" or 4" of the same soft foam would isolate you much more from the firmness of the support layers and you could then have a mattress that was both very pressure relieving (comfort layers) and very supportive (support layers) for many people.
Some of the firmest mattresses are like this where they have a couple of inches of very soft foam over a firm innerspring or firm foam and they would be “extra firm” mattresses even though they contain some very soft foam on top.
Post #2 here includes the factory direct manufacturers that are within 50 miles of you. All of them except Denver Mattress make memory foam mattresses although not all of them may be in your budget range. Original mattress for example only makes one very high quality memory foam mattress (at least as high quality as the upper end Tempurpedics) but they are @$2000 for the set (with a box spring). Their websites and a few calls to talk with them (and factory directs are usually quite happy to discuss your needs over the phone) will help you choose the ones that are most worth a visit.
There are certainly some better quality options in the list.
Hope this helps 
Phoenix