It’s done intentionally, it’s a classic confusopoly market, like cell phone plans. When your cost of goods is nearly fixed regardless of what you’re selling, you have to resort to games like this. It’s the same reason a handful of popcorn at the movies is $5 and an extra extra large is only $0.25 more. The labor cost for both products is the same, and the difference in cost of materials is about $0.02. No matter which one you buy it’s nearly all profit, but you feel like you’re getting a deal when you buy the large. Car makers do the same thing with options packages, options are almost pure profit.
Once you’ve decided which firmness you like, there is very little to differentiate one mattress from another apart from firmness. The cost to manufacture a “high end” mattress is only a few percent more than it is to make a cheap mattress: a couple extra springs, a little more foam, organic cotton, 5 minutes longer on the line. Assembly can’t be moved offshore because shipping costs would be too high, the process can’t be automated, so nobody can get a big cost advantage. So instead they baffle you with bullshit knowing that most people will just give up and choose the “mid range” model.