Surface, peak/decline of course.
But also Fed came back from mono in 2008 (lost to Fish and Roddick, and had not played in february in 2009 (withdrew from Dubai, I don't remember which problem he had) and 2010 (lung infection).
And he has not done that great in summer tournaments either
Nadal doesn't like Cinci and has his peak period finished after Wimbledon. Djokovic is usually fitter in march than in summer.