It's funny what people see in things. The truth of the matter is since Ricky's arrival (or actually, since they broke him in a bit) they have been on as near to a strict mathematical regiment of rotation as is possible. Ronnie gets two drives, Ricky gets two drives, then reset. The only exceptions to this rule are on any given drive if one of the RBs receives too much work he has to get spelled by the other one in the short term.
Look at the TWO drives prior to that final drive where you point out that Ricky Williams got the ball. Tell me which running back you see carrying the ball.
That's right. Ronnie. It's the puff, puff, give offensive rotation. Ronnie takes two drives then Ricky takes two drives.