Actually it is down to Fifa having better gameplay. PES has been stuck in the Past and never moved forward.
Where as Fifa has just moved forward time after time and Fifa 13 is no different. Just watching off screen games of it you can tell the new A.I run system works, the new theme of unpredictability is there and tweaks to the collision engine are obvious and the new 360 movement/dribble thing.
Add in to the tweaks to Career mode, Ultimate team and other aspects and it once again pushes the football gaming scene forward.
Obviously though the true extent of all this will be know on September 11
PES 13 is the first PES (going by the first demo) that I can actually play since PES 6. The rest have been horrid, yes, Horrid.