It's also the reason we have bifocals, trifocals, and "progressive" eyeglasses. It is the reason that some photographers find an increasing need for and reliance on using a loupe to focus the ground glass image (just kidding, but at least partly true). I personally use progressive glasses and any other means of focussing and clariying that I can find.

The more dilute form of the developer is more likely to slow down in the highlights and allow continued development of the shadow areas as I understand it. I used to use dilute Rodinal of Neofin Blau for this effect.