well + Maximum damage just adds to the highest maximum damage of the player ( i/e: you do 100 - 400 damage total,add a 15 max jewel, you now do overall 100 - 415 damage) whereaas with ED%, ED% affects ALL damage including minimum/maximum ,so the actual max damage number doesn't change much. ( i.e: 100-400 damage ED% would add to the minimum damage AND the Maximum damage ) this would make a + Max Damage jewel/rune/ Whatever APPEAR to give more damage, simply because it only affects the overall maximum damage shown. ED% adds to both min and max damage,thus giving more overall damage.
if you do 1-1000 damage, you have a CHANCE to only do 1 damage per hit... so raising your minimum damage shortens the gap in the damage that you do. ( Ideal is to have your min-max VERY close to each other, so you deal an average of more damage per hit.
if your min-max is ALREADY very close,you DO NOT want to be adding only max damage jewels/runes/etc beause that would increase the gap in your min-max damage. This means an ED% jewel would work MUCH better than a +Max.
sorry for spam,bud damn nice find... WTG man