This. And by now, he's just better at the classics than Wout. Not by a huge margin, but he is.
I suspect that's also because Wout has turned himself more into an all-rounder because that's what his team needed from him.
Mathieu has always been in a team where the GC doesn't matter. He is the star in the classics, Philipsen sprints in the Grand Tours.
Had Wout been on another team where he could solely focus on the classics, things might have turned out very differently, but as it is, let's not forget he also pulled off some ridiculous stuff in support of his main GC guys. In that sense, he's actually quite unique come to think of it. But it's all speculation anyway.