Are you saying the front edge (bottom?) of the individual keys is angled differently than the other three sides?
Its to prevent the keys touching the screen. With older macbooks typing left a small residue of gunk on the keys, and when you closed the macbook - even though the screen was inset and shouldn't touch the keys, the gunk would come off onto your screen, leaving little shadows for each key.
The inset keyboard prevents that while keeping the lid and screen assembly as thin as possible.