Not sure how it is on iOS, but I'd assume your app has to deserve to some extent to be in the top. If you're in the top because of a fluke (e.g. you got featured in some blog post), you'll fall as soon as the hype fades. No idea really, the most "high-ranked" I've been on iOS was around rank 80 Top Grossing in the Productivity category, which I lost as soon as the launch hype faded.