Put yourself in the ref's shoes for this exact incident. He might see that *something* happened, but he might not have seen exactly what did happen. If he's not entirely sure that Sergio swung an elbow with the intent he apparently did, he can't punish him for it there & then but he can go back and see what did happen.
I understand the point from blues that if he was wearing a red shirt the outcome would likely be different, see Fellaini, Rooney etc consistently escaping punishment.
Jacob, I will just reiterate a point I made a couple of days ago. Andre Marriner is a professional referee, he is paid to make split second decisions whilst under pressure, that is his job. The fact that he may have made a mistake with Aguero, and most of us accept he did, does not alter the fact that he saw it and that, under current regulations, is one of the criteria for retrospective action. Whether we would have 'dodged a bullet' if Aguero hadn't been banned is immaterial. If you cannot trust the FA and referees to administer rules to the letter of the law then it's a worrying situation for all clubs, that is the bigger issue.