It sounds strange but I think the difference is that Byrne drew almost 40 issues of the other X-Men characters, and most artists since him seemed to be looking back on Byrne's character designs for them.

With Kitty, she'd only just been created and there wasn't as much of a backlog of material that he'd drawn her in, especially since her only "costume" at that point was the yellow-and-black training outfit. So when future artists worked on the book, they only had a tiny bit of Kitty material to look at. The other artists's interpretations of the character became just as much an influence.