This was my thought as well. If a melee character has a weapon skill of 400 and the boss has defense skill 415, then the melee would predictably see a 0.6% drop in their crit chance. It's also possible that applies to casters as well (giving a caster a casting skill of 400) and that the crit reduction also counts against spells. The numbers don't seem to add up (a predicted 0.6% crit drop versus the ~3% people are seeing), so there could be a second formula at work, or the numbers could be inflated for some reason.