One of the main reasons for the two separate inventories is for room maintenance and upkeep.
DVC only sells about 97% of the inventory, keeping the remaining 3%. Here's a simple example:
Say a resort has 100 villas, and they sell all the time for 97 of them, retaining the other 3 units. This leaves 97 villas available for points, and 3 for cash.
Now they need to do maintenance on some villas, so let's say this week they have to pull 2 units out of service. This still leaves 97 villas available for points, and 1 unit available for cash.
The object is that there are always 97 units available at all times for points members. When the other 3 units aren't needed, they are made available for cash.
That's the difference between declared inventory and undeclared inventory.
Now the 2nd part of the equation is when members use their points for something else, outside of a DVC resort. Say you use your points to go on the
Disney Cruise Line. DVC needs to pay
DCL cash for your cruise. They do this by taking your points and applying them toward a villa. They then rent the villa out for cash, and take the cash they received to pay for the cruise.
So while it looks like there are cash reservations available, it really means that the unit was reserved using points (so it's member's inventory), but is being rented by DVC for cash as part of the 'exchange' between DVC and DCL.
So in our example, let's say that of the 97 DVC villas in member inventory, 5 members used points for DCL, the Disney Collection, etc. Say those 5 member's points were enough for 5-villas for a week. So when you call to make a reservation, instead of 97 units in inventory, there are only 92 units, with 8 units available for cash.
For December it's probably just the undeclared inventory that's available for cash. DVC does not normally use units in prime times (DVC-wise) to rent out for cash to pay for those members who used points outside the DVC-resorts themselves.
Hope that makes sense.