I wonder how iTunes rankings work

I have a script that watches my iTunes rankings for me. Auto Adjust is always in the top paid iPad photography apps and usually the top paid iPhone photography apps. While the iPhone ranking is always lower (there are way more iPhones and way more iPhone only apps) they don’t always move in the same direction. I’ve found the following bizarre trend to be true.

When Auto Adjust is top 50-100 iPad, it’s somewhere in the 300’s for iPhone.

When Auto Adjust is in the lower 200s for iPhone, it’s in the lower 100s for iPad.

I suspect this has something to do with the “rolling” nature of the rankings – they’re a function of how many times each app was purchased in the last (probably) 24hrs and each time an app is purchased they shuffle a bit. The photography category is low traffic (compared to games for example, which needs its own app store at this point) so I imagine this increases the volatility of the rankings.

The most bizarre disparity I’ve seen is iPad:87 iPhone:400+ (the rankings only go up to 400).