100/90 = 1.1111
350/315 = 1.1111
468/421.2 = 1.1111

The damage difference would be static regardless of level/resistance. Now, if it really is a 15% increase instead of 11%, then yeah something's probably wrong. And I don't disagree about the magic/non-magic bit. Just trying to work out the percentage stuff, but I haven't seen any testing on it.

Also, I could be doing my math wrong. /shrug