So there are a couple of different things going on. The "Leader Board" is by reputation points (from people liking posts) and defaults to the "by month" time frame, the "Top Members" list and the ranking titles that show up in profiles (the "shark", "zoanthid" etc.) are based on post count only and are cumulative. You can see the cumulative reputation points on the profiles and the Top Members listing as well but that isn't the sort in the latter case.