seems to me the formula is the same for AR and armor with only a factor 10 difference.
They each have dimminishing returns so when you compare your AR*10 with armor its best to increase the lowest.
280 (equals 28AR) armor vs 78 AR is a huge difference. Not only is AR usually lower anyways, even if it is not, it would need to be an enormous amount higher to make the armor beat the AR.
My guess is that it feels like its going better because you kill stuff faster so that they have less time to damage you. ?