Federer can be consistent when he wants to be. Watch RG 2011 semi where he served like Jesus on crack.
He beat Nole from the ground for a majority of the first half or more of that match. Not saying the serve didn't play a part.
Answer OP question: It's because he has a lot of tools at his disposal and he can pick apart the lower ranked guys with obvious flaws in their game. He'll still be streaky but he can ride it out. His slice/drive backhand shots keeps that wing from being a liability but it's exposed by the top players especially obvious name #1 and obvious name #2.