5 agility is kinda negligible, some fights you take spell dmg so having more hp is kinda better. possibly what I am thinking is this guy's guild bank is out of Shadowsong Amethyst's or he had an extra blue gem lying around and decided to stick it in. The tank is geared to a point where it doesn't matter about 20 hp and 5 agility.

But yes, If i was this guy I would have put a diff gem in.
Btw we are referring to is this gem: http://thottbot.com/i32212