I'm sure somebody could upload a clip, but basically Aguero played the ball into Sterling who was 2 years offside. The defender reached to catch up with him and the ball deflected off him onto the shooting foot of Sterling who scored.
So on a tenuous technicality, they ruled that the defender was the last person to touch the ball before Sterling scored, thereby ruling him onside. But the defender would never have been in that position and made the challenge if Sterling wasn't in an offside position in the first place.
A bit like Kane against Arsenal last week, Mustafi fouled him, but only because he was racing to try and get ahead of him due to Kane being in an offside position.
The rule on offside should be clear for me, if a player is in an offside position, then it's offside. Whether he tries to play the ball or not. The moment the lino sees an attacking player ahead of the last defender when the ball is played, he should raise his flag.