I copies this from a post on d2jsp addressing this bug and explaining exactly what it is.
Apparently this has been an issue for a very long time:
Case 1, item has innate +%ED. You socket with a jewel with +max or +min dmg, or both.
Example: You have a circlet/coronet/tiara/diadem with xx%enhanced damage, or a paladin shield with xx% enhanced damage, and you sock a +15 max dmg jewel.
No bug! Will work as intended.
Case 2: You socket an item with a +%ED Jewel. Then you socket another jewel with +max or +min dmg, or both.
Example: You have a body armor with 4sox/100life. First you sock a jewel with 40%ed. Then you sock another jewel with +10min and +18max dmg.
No bug! Will work as intended.
Case 3: Item has innate +max dmg. You socket with a +%ED Jewel.
Example: you have a circlet with +12max dmg. You sock a +40%ed and 15%increased attack speed jewel.
Bugged! The %ED only applies to your overall min dmg, not your max dmg
Case 4: Item has innate +Min dmg. You socket with a +%ED Jewel.
Example: you have a circlet with +12min dmg. You sock a +40%ed and 15%increased attack speed jewel.
Bugged! The %ED only applies to your overall max dmg, not your min dmg.
Case 5: You socket item with a +Max Jewel, then you socket with a +%ED Jewel.
Example: you have a diadem with 30% faster run & walk and 3sox, you first sock a +30max dmg jewel, then you sock a +40%ed and 15%increased attack speed jewel.
Bugged! The %ED only applies to your overall min dmg, not your max dmg
Case 6: You socket item with a +Min Jewel, then you socket with a +%ED Jewel.
Example: you have a diadem with 30% faster run & walk and 3sox, you first sock a +18min dmg jewel, then you sock a +40%ed and 15%increased attack speed jewel.
Bugged! The %ED only applies to your overall max dmg, not your min dmg.
Case 7: You socket an item with a %ED/Max Jewel. The %ED only applies to minimum, not maximum.
Example: you have a diadem with 30% faster run & walk and 3sox, you socket a +40%ed +15max dmg jewel.
Bugged! The %ED only applies to your overall min dmg, not your max dmg
Case 8: You socket an item with a %ED/Min Jewel.
Example: you have a diadem with 30% faster run & walk and 3sox, you socket a +40%ed +10min dmg jewel.
Bugged! The %ED only applies to your overall max dmg, not your min dmg.
Case 9: You socket an item with a %ED +Min & +Max Jewel.
Example: you have a diadem with 30% faster run & walk and 3sox, you socket a +30%ed +10min +18max dmg jewel.
Bugged! The %ED doesn’t have any effect at all, as it will not apply to either your overall min or max dmg (worst case scenario)!
Any chance this will ever get addressed?