This doesn't make me wonder at all.
The US and China were the two biggest allies in World War 2, became economically unified during the cold war to defeat the Soviet Union (remember the Soviet Union?), and now are the two biggest economic powers in the world. Don't look at official GDP figures, they're meaningless. Look at influence, manufacturing/logistical and financial power centers, and purchasing power parity.
After all, what is China but a United Provinces of Asia? You think 1.3B people are naturally part of the same country/culture/creed/flag? No fucking way, to think otherwise is racist ignorance.
Japan is quickly being usurped by Korea as well. Once the re-unification of Korea happens and there is a large source of inexpensive labor from the North for the developed economy in the South to employ, Japan will start its relative decline. They've already started to use Chinese manufacturing to remain competitive and leverage what's left of their engineering/design head-start given to them by the Marshall plan and previous Imperialist mindset.
The big picture, folks. It's always worth looking at.