X & Y skew is not the same as keystoning. The problem is when your projector is not centered on your screen. The pixels will be closer together on one side, so you may correct the keystoning, but the pixels will not be proportionate. The skew squishes the content to either side to make it look like it's normal(yes... 'squish' is a very complex technical word... note sarcasm of my own choice of words). What video folks will roll their eyes at is that you should have done the math to get a straight on shot. The other is that you lose detail in the image because you sample some pixels and stretch them/squish them.