Obsidian Arrow not affected by other player Earth attunement

seems that the Obsidian Arrow is not affected by opponent earth-attunement...
2 players with a great earth attunement (70-80) can do a 200-250 point damage each other... (using stacks)
but i think that thebackground math of an hearth hit must be something like (
oversimplify everything):

hits=(attackerattunement-attackedattunement)*attackerlevel/10;
if (hits<0) hits=0;

but this seems not happens with the obsidian arrow...

 

I think that equation changed with the release of resists, il try and find out.