Nice observation.
Well, I guess there might be a scientific reason whether in real life or in the world of Runeterra for the hip spoilers/blades or whatever you want to call them, but I can't fathom why they did that to Kayle's shoulder armor and your observation seems to hit the nail on the head. I don't think they were thinking too hard about how practical it would be for Kayle to have that kind of armor on her shoulder but if they did then they seriously didn't care if it looks super unrealistic to anyone that would imagine having to wear that stuff. It looks to me like maybe they just tried to 'make her look like this' as you are presenting these pictures side by side. I never really liked this Kai'Sa skin anyways.
I guess this proves that they rushed the designs or that they really don't care if champions look similar to each other. The Kayn legendary skin looks like Ekko(not default Ekko of course, but one of his skins...BTW, default Ekko has aged poorly) because they changed his animations so drastically.
I'm okay with artists coming up with something and not changing it in general, but when you need clarity in a video game, it's a shame that the philosophy goes where it doesn't need to. It's completely understandable to make mistakes. And a lot of art is meant to be at least a little bit inaccurate simply because we're quite imperfect. But when you have digital art going on, it's expected that you make changes to something that doesn't make sense.