I think it was some of that, and also, Mike realizes Moe is a hothead, likely to blow up (which of course, he does to some extent when he yells "I'll buy YOU out!!") and he's just mollifying him to some extent to get him to calm down for a minute and defuse the situation.

Remember, at that moment, Vegas is Moe Greene's home turf, and Michael had to tread at least a little bit lightly. If Michael pushed the situation to an open confrontation with Moe, by openly accusing him of skimming off the top, somebody would have to do something.

Better for Michael to throw Moe a small bone ("you're not skimming, just unlucky") to defuse the situation before it came down to a direct call-out. Michael already knew he was going to get rid of Moe; he just wanted to do it on his own time frame.