Notice how in KH1FM and KH3D Xemnas uses the same moves in both games, while in KH2 a couple of the moves he used in KH1 and 3D he didn't use when he fought Sora in KH2. Same goes for Ansem SOD, when you fight him in KH3D he has a lot of moves that he didn't have in KH1. Now why could that be?
My theory is that, since Sora and Riku can't kill Xemnas and Ansem SOD in KH3(Otherwise, that would drastically change the timeline, since they were killed in KH1 and KH2, and can't be killed before they return to their respective times and fight Sora and get killed in those games) they will end up battering Xemnas and Ansem SOD a lot and end up knocking them out, and then when Xehanort is defeated, they go back to their respective times. That way when you fight them in 1 and 2, they are weakened from the battles in KH3, but due to the laws of Time travel in KH3D, they wouldn't remember that they got weakened, so they would fight Sora without knowing that.
But for that to make sense, Ansem and Xemnas would have to be alot stronger when you fight them in KH3 than in 3D in order for them to still be powerful enough to put up a decent fight in KH1 and KH2 aswell.
Notice how in KH1FM and KH3D Xemnas uses the same moves in both games, while in KH2 a couple of the moves he used in KH1 and 3D he didn't use when he fought Sora in KH2. Same goes for Ansem SOD, when you fight him in KH3D he has a lot of moves that he didn't have in KH1. Now why could that be?
My theory is that, since Sora and Riku can't kill Xemnas and Ansem SOD in KH3(Otherwise, that would drastically change the timeline, since they were killed in KH1 and KH2, and can't be killed before they return to their respective times and fight Sora and get killed in those games) they will end up battering Xemnas and Ansem SOD a lot and end up knocking them out, and then when Xehanort is defeated, they go back to their respective times. That way when you fight them in 1 and 2, they are weakened from the battles in KH3, but due to the laws of Time travel in KH3D, they wouldn't remember that they got weakened, so they would fight Sora without knowing that.
But for that to make sense, Ansem and Xemnas would have to be alot stronger when you fight them in KH3 than in 3D in order for them to still be powerful enough to put up a decent fight in KH1 and KH2 aswell.
Thats my theory.