First of all, I would love to see the lifegiver heal more, that's for sure.

On the defense of STS though,
1. "According to my research", lifegiver heals 3.2-4.0*DMG. You ask for it to be derived from health instead, but I'd say the way it is now is reasonable as well, and works throughout most of the game.
2. Rogue's heal only works for 1 person while warrior and sorcerer heal all people in range, so the warrior's heal is the best one IMHO. Which is to be expected, considering their tanking/protector role.

Why the numbers you provide, 1553-1941, work for DMG 485 and not 533.8, is a good question, though. Could be a bug with arcane staff. It should heal some 200 more.