Ask HN: Huge variability in App Store rankings?

Hi HN,

My first iPhone app has been on sale since less than a week, and I've observed huge (and seemingly inexplicable) variability in its ranking on the AppStore.

See for example this chart (thanks to appfigures.com) of its ranking in the Navigation category of the French AppStore:

http://i.imgur.com/YRktW.png

One thing I do not understand is the wide swings and sudden drops. Are there any experienced iPhone developers/marketers here who could shed some light on these strange artifacts?

Thanks!