Unless you're download location preference is the Curse one, it will go for whichever download is available first, from what I understand. And sometimes packs push to CreeperRepo first, sometimes Curse first.
But I believe, because Curse is having to do a lot to keep up with all the stuff that we (as a community) are throwing at them (bug reports, mod packs, downloads, etc.) that may be something to do with it. But then again, I could be talking out of my butt, as I don't know the ins and outs.