Dave
With regard to your ancestor serving in one cavalry unit yet being listed as AWOL by his original unit -- something that frequently occurred -- the answer may lie in the fact that according to the Articles of War, a man absent from his unit without the permission of his commanding officer could be punished by court-martial. Given that, I would interpret the muster roll entry regarding your ancestor as a notation that he was AWOL because he did not have the permission of his commander to be absent, and that the unit had some factual knowledge or guessed/assumed he had joined a cavalry unit without the permission of his commander. So he legally still belonged to his original unit and was in violation of the Articles of War.