You need a better definition of what flat design is.
flat and 3D are NOT mutually exclusive.
This thread is not about flat in general but specifically about "iOS7-flat."
Apple has made it clear that they see "iOS7-flat" as apps that are painted on glass layers without any illusion of depth applied to them.
Google sees it differently.
The original question was "why does Google see it that way?"
I have no freaking clue why. I can't answer the OP.
But people need to stop saying that it's a not a valid question because there's no difference. There clearly is a difference between how Google and Apple sees flat design.