I’m not positive, but I think the game considers Life Leech and Energy Leech to be “damage”, hence the reduction in those resistances. I’m guessing they are classified as secondary resists like Aether/Chaos resist…
Energy Leech resistance doesn’t get a penalty in Ultimate though.
I do wonder though: does Energy Leech resistance reduce energy burn damage? I would up that shit if it did just to be less annoyed when Fabius or Sentinel spawn with an energy burn weapon.