Say, if an enemy makes a kinetic attack, of 50, and I have 50% of kinetic resistance, and 9 of defense:
50-50%=25
25-9=16
I'm gonna suffer 16 instead of 50. And without any kinetic resistance
50-9=41
41, but only to my HP. If defense takes no effect on shields, that makes it much less useful than general resistence. For example, if I had an armor with 2 defense but a 50% kinetic resistence, I should pick it over one with 9 defense and 20% kinetic resistence bonus, since:
50-50%=25
25-2=23
50-20%=40
40-9=31
it'll give me more overall protection.