Husbands 1st semen analysis yielded 15mil swimming sperm (6mos post vascet reversal). 12 months later (12/13), 6mil. can tests results vary like this?

Yes: Variability in results of semen analysis are often more the rule than the exception. Many factors can affect the results and cause variability from one semen analysis to the next, including but surely not limited to: duration of abstinence, timing, collection method, processing by lab, natural biological and even seasonal variation, and the possibility of scarring at site of vasectomy reversal.