Experience, more mental game etc.... etc....
Ask yourself this, Fed 05 vs Fed 13, who wins? Even if Fed 13 knows what's coming, there's nothing he can do about it, once he has got that ball back, another one is fired cross court with ridiculousness pace, and all Fed 13 can do is a little chip back, if anything at all.
No one will ever play tennis as good as that ever ever again, the problem is a lot of people posting on these boards now never even saw it, so they think it's some sort of exaggerated myth. I assure you, it's not.