NTSC machines can execute less operations per frame than PAL machines due to the shorter duration of frames (16.67 ms instead of 20 ms). Therefore, NTSC machines are not able to execute all the operations that the game requires each frame, thus causing a frame skip every other frame. The end result is that the game runs at 60/2 = 30 fps (and sprites flicker here and there due to the lost sync, and music gets slow down). On PAL machines, instead, the game never skips a frame, so it runs at 50 fps.