[Stepsongrapes]

Yeah. Basically, if your archer unit (hunter or otherwise) has an attack value that stacks with your modifier to yield a modified attack that is over 30, you're losing the full affect of dragon arrows. The skeletons, having a lower unmodified attack avoid suffering from this or suffer from it less than a hunter.

Roughly speaking, each point of difference between defense and attack yields a 3.3% damage increase, capped at 300% (3X base damage). Dragon arrows drops the target's defense to zero leaving a higher chance that you will hit that cap.

EDIT: you can test it yourself by repeating your black dragon kill test with the hunters after shedding most of your +attack gear. Watch the damage numbers and you should see a bigger difference, with and without the dragon arrows.