I don't think today - and to some extent, quite a few games in the last month or two - should be much of a surprise. We bought him at a time when top strikers were at a premium. I think you'd be hard pressed to argue that he's top class. If Lukaku never improved from here on in, he'd still be a perfectly serviceable, second tier striker for us, albeit one who is prone to fairly pronounced peaks and troughs in form and who, at the very least, could be said to have a no-better-than-middling record against good sides. He scores goals in bursts and isn't really going to win you that many games through moments of individual brilliance.
All that said, give him support and good service and he'll score plenty of goals. And it's easy to forget that he's only 24. I don't think he'll ever be the most subtle or clever of players, but he can develop his game and find ways of adding facets to his game that will make him more difficult to defend against and add a few more goals to his game when he's not playing well. And, if we somehow decide to stop cnuting the ball at him from 70 yards away, well, that might help too.