How do Windows Phone Charts work?

Has anyone some details about the rankings in the Windows Phone Store and how they work? It seems to be impossible to find better general information about it.

For example I released 3 games on the wp store. One paid, two free. The paid one has sold 2 units in the first 2 weeks. The free were downloaded each like 15 units in that same time period. The two sales made the paid game climb in the rankings (Top 100?!), even in contries it was not downloaded. 

With one of the free games the situation was the same. With that few downloads it reched the rankings. The other free game (same category btw) saw absolutely no rankings at all.

Next thing, while the paid app was still in the rankings of some contries, I switched it to free for some days. It received nearly 4000 downloads in that time, but was never in any rankings? Switched it back to paid, got another 3 sales, never reched a chart position.

I’d be interested to read some of your experiences or advice with WP store or some links to more information on that topic.