Some youth programs are just more succesfull than others. When you compare it to Club Bruges, who is a major club in Belgium, their youth academy hardly represents anything in terms of players brought forth.
I don't really know a lot about how these academies work, you might want to ask Steve about it. But my guess is that there's only 2 or 3 teams in the belgian highest division from the french speaking half of Belgium. One of them turns out to be one of the "big 3" in Belgium: Standard. Maybe that way it could be a reason for half of the country's promising youth to come to Standard. They brought forth Mirallas, Chadli, Fellaini, Witsel, Van Buyten, Benteke and a few more of those. Anderlecht on the other hand seems to attract a lot of Dutch speaking talents like the Lukakus and Dennis Praet...
But I repeat, it's just a guess, ask a second opinion.