Sounds like the good folks at Weber haven't got a clue as what they're doing or why. They basically said, we had a our reasons, but even we don't know what they were.
I'm doubting that they made a bunch of different sized prototypes (16", 17", 18.5", 20", 21",, 22.5", 24". etc.) and then l;oaded them up which charcoal and cooked a bunch of steaks on them.
I like Troy's idea.
BD