is 100% PDR really is 100%?

that's his issue, he has 100% ignore, which is the same sacrifice has, albeit using a different method.

it shouldn't matter, as long as i have 100% ignore, my impale should hit the same in a snail as it should in castle

There are some skills that ignore WDEF.

IIRC Shadower's (old) Assaulter ignored all WDEF on a target. The same is with Sacrifice + it completely ignores PDR as well.

That small difference in damage could just be from the mob's WDEF reducing his damage... unless he's using Sacrifice.