But in that case, presumably your assignment would go to someone who's versatile in writing slash rather than to someone who prefers to write het.
I think the idea here is that added pairings increase the mods' chances of making good assignment matches -- and the increase is more significant if there are a good number of options in any one category rather than simply across categories. Argh. I see the math in my head but can't find the lingo to explain it properly. This is why I'm in liberal arts! LOL