Apple design still have gradient, layers of transparency and text shadow to show depth. How is it flat? Well it is less skeuomorphism but I don't think that means flat.
I agree Microsoft's metro is flat, no shadow, no gradient and feels everything is on same plane.
It was just rumored that Apple would move to a flat designs before the keynote, and people just stuck with it.
Just like everything else, software has fashion trends. Right now, flat is in. Once people get used to flat, something else will appear with the "innovative" look.
In last few months all I see is FlatUI in news & the way its changing everything. It is all hype, without understanding some associate it with anything they see "minimal" now, to the extend of saying iOS7 is flat design.