Factor in the extra DLC sales that come from having a larger install base on the XBL and PSN networks and the PC gets dwarfed even further in terms of revenue as the publisher gets a lot larger share of the DLC sales than they do for the retail sale.
I know you're talking about Assassins Creed but just wanted to say that the PC probably has a lot more digital customers than the consoles do. Steam has 35 million active accounts, the conversion rate of console users who do stuff on XBL and PSN probably isnt even close to 1:1.