Why is leaderboard position so variable?

I'm confused by the leader board position changes of my blog. When I first added it to the network it was number 2 (which was a little strange for a new blog). Then after ~week and yesterday it was #3. I posted my third post in a week and this morning it was #23. In a group with only 27 members with little activity this seems like a strange shift in ranking. Another blog that was #2 yesterday is now at #24.