No idea how he could have thought that, but then again I also have no idea how he missed this this. It's a free kick. Nobody's moving particularly quickly, and Aguero set up in an offside position about 10 seconds before the kick, then stayed there. It was never close. I did notice that the assistant was about a yard inside the touchline (on the pitch). That still shouldn't affect his judgment, because the goal area line and the cut of the grass are horizontal and he can use those to orientate himself, but it was still odd.
He also blew a very clear throw-in decision in the second half. I'm sure he found out he was wrong on the City goal fairly quickly, and was a mental wreck for the rest of the game.
I thought the AR might have thought toure made the header, as they were pretty much in line at that point and if you had the two of them next to each other, what are the odds it would be augero?
Didn't we just debate something similar in the MLS thread regarding the Chi/NYCFC match last week? It is thought that since Aguero made a challenge for the ball, that would be enough for it to be offside.