In my opinion, once you get close to the ceiling, the improvements you make to your play have smaller and smaller returns, hence the small difference between plat/diamond DPS. But since the healer ceiling is so high, even platinum healers have so much room for improvement that can severely influence the game, like fluid aura usage, perfect predictive benediction timing, GCD-mana management, etc. The gap isn't because the floor is low but because good healers are too good at differentiating themselves.

The gap we are talking about is the same, but the cause we are thinking of is different.

Sorry about the double post and weird apostrophes. I'm on mobile.