There are a bunch of different ways mobs hold out their arms.
1. The way the player holds them-- down
2. the way zombies hold them--straight out.
3. the way a player holds them when drawing back their bow.
(there are also others of course)
The bug is that Skeletons use the Zombies way of holding their arms. They are supposed to look like they are drawing back their bow
MC-1956 Arms misplaced in skeletons with a bow