From this picture, can you see the need for less stagger on a banked track? I use 25° only for illustration purposes, I doubt there's many tracks with that much banking. Anyway, at 25°, the path's each wheel is following are closer together, so less stagger on a banked track. If I keep on increasing the degrees of banking, the difference gets smaller and smaller. If the banking was 90°, can you see that there would be no need for stagger. Stagger is greatest on a flat track and gets less and less as the track incorporates more banking.