As previous posters have indicated, there are differnces in inventory, and sevelal ways that Disney/CRO gains control of rooms from DVC Inventory.
1) Developers Points...these are points that are available for sale to DVC members, but are not yet sold. These come from ponts garnered by Disney through ROFR contracts, or in the case of SSR, buildings that are finished, declared into DVC inventory, but not yet have points sold. Developers points can be used for promotions for sales, by CRO to sell for cash, or for upgrades from other WDW Resorts.
2) Member Trades...When a member trades out to any location, except through Interval International, a room is turned over to CRO for cash to offset the cost of that trade.
3) 60day unreserved inventory...If DVC Owner rooms are not fully reserved by DVC members, the inventory is turned over to CRO for cash reservations. These rooms, if not rented, can also be "reclaimed" by DVC Member Services for members...but they are probably the least likely source of CRO Rooms.
4) Retained ownership...Disney/DVC retains ownership of a small percentage of rooms (estimated at 4% to 6%) for maintenance and "emergency upgrades". If unneeded for maintenance, CRO can rent these for cash.
The only one of these can can be re-claimed by Member Services is the 60day turnover, otherwise, once a room is in CRO inventory, it is not available to DVC members on points.