That's how the image gets drawn on screen in those games. Remember the GBA's screen draws images differently than a TV would and in all essence the GBPlayer is still drawing the images the way it drew them on the GBA, with the exception of the obvious hz difference between the two of them. Put it on soft, and the screen will become blurrier, but it'll hide the skip. Put it on sharp and you'll see it after a certain amount of screen refreshes. It's completely normal and nothing is wrong with your GBPlayer. I remember noticing it at first myself, but now I don't even pay any mind to it.