It's an anomaly in the most literal sense yes, given that he's only managed >10 league goals once in his career.
At the same time his 18 goals in 87 games for Belgium lead to 0.207 goal/match. A guy like Wesley Sneijder, who actually was a shared Golden Ball winner once, is on 31 goals in 134, or 0.23 for the Netherlands. The mythical Steven G. is on 21 goals in 114 games for England, or 0.18. And then Fellaini has been subbed on or off a lot of times, so I wouldn't be surprised if someone did the math and concluded Fellaini actually has a better minutes to goal ratio than those two, and also tops various other high profile midfielders.
You were talking about crossing earlier and that might genuinely be one of the explanations, because it's easy to single Fellaini out, but he's far from the only player who I think has been underperforming. Maybe in his last season at Everton it really clicked with a guy like Baines or something leading to a bit of a purple patch, and with Belgium he's of course surrounded by lovely players who create so much. While over here the level of crossing has indeed been atrocious over the last 5 years and therefore he's been unable to benefit.