The USO Series plays a bit slower than much of the post USO season, its bounce is generally higher than those events and he tends to use the sun/heat/wind of outdoor events to his advantage whereas much of the tour prefers the sterile conditions found indoors. The 2013 USO Series also happens to be something of an outlier for Nadal as it's probably the only year he's been by far the best player on tour during that period. Seriously, do you follow tennis closely? Why should it surprise you that Nadal does relatively poorly during the indoor-like East Asia and indoor Euro period of the year?
And how has Nadal been the only player peaking for slams? Fed's been doing that for years now besides his universally shit 2013, Murray's made a habit of it recently and all-time greats like Sampras did it late in their careers too. As I said previously, Djokovic's poor slam record relative to what he's achieved during the rest of the year is what's unusual, not Nadal's peaking for slams.
Ya, I knew you would come up with "explanations". You are acting like a lawyer who believes he has a guilty client, trying to "argue away" the truth.
Before 2010, most Nadal fans rationalized that Nadal couldn't win the USO, because "it is too hard on his knees", and too quick, and low bouncing. Now that he has won it twice, "his knees are cured by a process that has been clinically proven to not work, and the USO is slow and highbouncing". LOL !
You don't get to the "truth" by arguing it away, or any other form of denial (closing the thought process). You get to the truth by "opening your mind to new thoughts".
Try it some time.
By the way, here is Andre's "conditioning coach". Notice the bloating and the acne.