Price-point marketing. You look at a demand curve, and it's either a straight line, or convex. But you think of it as Price-Pointing, the line kind of bumps, with peaks and valleys.
Defenses are like that.
When you hit 21st level, BOOM! Improved Defenses (and its ilk) suddenly increase by one. Your ability scores are all (well, the important ones) even again. You're in reach for +5 armor, which for heavier armors can lead to yet another +1 to AC.
There are lots of "Price Points" for defenses, and some of them are spread out, making it look like a straight line from a distance. But if you zoom in, you can really see the texture of the curve.
I just thought that was something interesting.