Apparently Doomguy carries a razor now.

To me, it simply seems that most hand-edits don't seem to take into account that only one side of the arm generally has hair. To illustrate, all sprites on the leftmost side of Marty's pic are "anatomically correct", so to speak -- the first two show the top of the arm (hairy), while in the bottom two, the underside of the arm (non-hairy) is visible.
The inconsistencies come when someone makes a hand-edit and accidentally uses the wrong arm orientation. The second hand from the left on the 2nd row is a prime example -- as you can see, the hairlessness and the muscle tone of the arm suggests the arm was copied from a pic that shows the underside (most likely the first hand on the 3rd row).
A bit of an inconsistency, but to be honest, it's one I've only made recently myself.