At the moment of the screen shot, the ball has left his foot. While it's only a split second, it is the difference between just on and just off. Your DCR is running right, while the three potential offsides are running left. Given that the margin between him and the offside players is so close anyway (given the angle I can't say for sure, but there doesn't seem to be much in it) that moment makes all the difference.
Referees in FM aren't infallible, anyway. Offside goals will be given, and goals will be chalked off that shouldn't have. It happens, it seems quite deliberate, and it's a good thing. Means you can have a good old moan at the ref - wouldn't be the same without it.