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.
On iOS you are much more likely to stay in the top if you first get there. I.e. it has it's own mechanics internally and externally because more people visit the iOS app store.
Also much more sites are looking at the top in the app store and so your exposure for getting there is many many times more.
Further more much more iOS apps are inherently social so that helps too.
The point is that being in the top doesn't mean what it means on iOS. On iOS you will be able to stay in the top purely by being in the top because people use the app store much more frequently.
On the Mac app store you being in the top is more a product of a lot of other things you do.
If you hit a nerve just like you did and I to some degree did you will do well even if you aren't in the top of the app store.
At least that's how I understood it, but perhaps I am putting too much of my own bias into it.