It's just the nature of how Stam/Armor interact. When you increase one, the value of the other increases. You should also find points when increasing your stam that armor value goes up by quite a bit as well.
I don't recommend you just use one. Have both of them in your bags and use them for different fights. Pillars are gonna be your best EH piece most of the time, but the 277 legs will be better for the select few fights with high magical damage.