Also, I wonder if he was healed because you used an Earth spell on the big, bad earth elemental lord guy. I realize that as the player, with no warning, that would be pretty annoying to see, but thematically it kind of makes sense that an earth elemental of his power would absorb and be healed by Earth magic.
This brings me to a related point.... I really wish that there was some way to see what special powers enemies have, while we are actually in the middle of fighting them. It would make it much easier to know if something is a bug, or a result of a cool special power we didn't know about.