No alba, thats the point of this thread. The TW ranking is as you say (OD gained as a member of the tribe stays with the tribe) however i felt this missed the subtleties of the OD for a newly merged tribe like BANG!?
So i figured, TW stats ranking is the sum of all players OD in the tribe (so when a player leaves, their OD is deducted from the tribes and vice versa when someone joins)
I guess this allows a different but equally valid measure of a tribes strength.