That XC1 statistic was true, but it's several years old. (If i remember correctly, one of PP's releases was announced as being the highest-ever seller, then it was retracted with a statement that XC1's lifetime sales still beat it - but it was selling much more rapidly. I think that game was Brave Soul.) I doubt it's true anymore. And even if it were, XC1 has been purchaseable in physical or download format for ... a few weeks shy of twelve years, apparently. With no interruption. It's also FAR cheaper than Jast's new releases at this point - hell, they want $11.95 for it now.
I'm curious where the Sagara Family stat came from, though ... But in general,yeah, from what Peter has said, yarugee's consistently outsold story-centric games. (I hope Demonbane bucks the trend, and i really hope Aselier does.) As for MG, I follow MG's "top 5" chart: it's dominated by h-centric titles. Granted, the most recent couple of games have been h-centric, and the listings should be dominated by recent games. But so is Kara no Shoujo, and it's not up there much at all.
However, Score (the brand) seems to be one of the heavy backers behind Mangagamer. There have been several Score branded games released, one of them even in the initial batch of releases. So I think there's more or less always going to be a new Score title in the works for MangaGamer, it's only a question of which one. Sort of like how there's always going to be a Da Capo game in MG's release pipeline somewhere, it's just a question of which fandisc.
Edit: OK, Score is a Nexton brand. However, of the Nexton brands, Score has the highest number of games released. So basically, what Lipp said - they're cheaper and easier to do - combined with the fact that they have easy access to Score because Nexton is one of the drivers behind MG.