Yea, it's based on xp earned within the last month(?) iirc. So clans that earn achievements will have spikes in rank, and newer clans tend to earn some achievements faster off the bat.
This isn't necessarily a bad thing, despite the faults in accuracy, as it encourages newer clans to push through the tougher parts of getting themselves established.
The idea is that over time it would even out so that the currently active clans will retain the high ranks.