Skeleton Arms

In minecraft 15w35b, i used /summon to summon a skeleton without a bow. it started chasing me, and i started to see at times for a split second, its arms would go down or behind it.
https://www.youtube.com/watch?v=Z1pY20eTl3I&feature=youtu.be
I still think that the idea of skeleton with arms down at first is so cool.
I litterately just noticed it