Thats easy. Caster got poor sale cause they got in Indiefort bundle week lateк and everyone knows about that.
Zombies was bundled later too, in Groupees and indiefort, but it was unknown at time they was offered on IGS, so results is way better.
Journey Down, as everyone said, most of peoples already have it from IndieRoyale. IGS release offered nothing new, so sales was poor. If they offered, in example, that superion OST they have everything will be much hotter.
Fotonica sales got a certain boot then link was posted on HumbleBundle blog comment. Linux users was angered with THQ bundle and got Fotonica instead, Thats how i got it too.
Its strange to me that Gimbal was so popular - i found that game pretty boring.
Syder arcade offeres good OST, almost 10$ worth, so they was purchased at least for that.
Anyway, its pretty obvious that game more or less not long ago bundled somewhere else, like IndieRoyale have lesser potential to be sold here. Poor bonuses means less interest. Presence of Steam keys not really boosting sales too.