I find in the bestiary that Skeleton Wizard has the pierce DR low than their general DR when Skeleton Fighter and Human Skeleton have it higher than their general DR.
I don't see a reason for this, so I decided to post. I am still quite new to the game though, I am sorry if it is actually intended.
(Well, some games have casters take more damage from marksmen, but I figured it is not the case for PoE.)
p.s. somehow I cannot upload my screenshot, it keeps failing