Kinda building on what ioi said is you have to think about the nature of a pre-order. Generally this is a representation of either veteran gamers or just huge mainstream games. I mean generally a game like Modern Warfare 2 wouldn't be preordered to that extent (hit or miss with veteran gamers) but because it is a huge mainstream game it was able to get numerous amounts to get ready for the game.
Another important factor is of course the nature of the game. Games which the core are generally going to buy are going to be ones that rack up preorders. Thus you see a lot of those represented especially when it comes to Wii, DS, and PSP. Also because PS360 have a well established mainstream buying base that has become very loyal, a lot of their games are getting reserved as well beforehand. Final component is just the nature of advertising. Most of the games on there have been advertised and talked about for months and sometimes years. They've had time to be heard and therefore going to get more reserves early. But for games that don't get that kinda word of mouth you will find they get more reserves a few weeks before they release.
So it has to do with veteran gamers versus mainstream/casual, nature of the game, and level of advertising. Only time those bounds break is given that PS360 have built a pretty loyal mainstream following akin to that of veteran gamers (just buy the next big thing attitude). But for the most part reserves are going to be about veteran gamer choice (core loyal fanbase) and big mainstream marketing projects like Modern Warfare 2.
Reliability is of course like anything but I'd say we are in the ballparks for most if I were to guesstimate certain preorders myself.
