App store rankings, page-204

Had to check this out, you're right. The grossing ranks frequently drop to the 20-30 range and then spike back up to 5+.

Upon further research, it appears that IOS market share in NZ is approximately double androids.

The other factor to consider is that the 2 operating systems quite often include a different range of apps in each category, and so are not always ranked against the same group of apps.

The important chart is the 'overall' rank which is still showing growth against previous month's.