I just ran several searches using the Extra Vacations, Exchanges, and Last Call option and noticed exactly what you're experience. I did not run the same search as you, so I did not note any availability you are not seeing, but I did notice what I believe to be the reason for the discrepancy between the number of units available on one screen, and a different number showing on the next:
Take as an example what I have onscreen right now in another window
I ran a quick search for any resorts in the US during July of 2011, and came up with more than 400 resorts with unit available. I looked at the first 2 pages, each with 10 resorts, and saw 7 had units available as both rentals and exchanges. I looked specifically at 3 of them, because the number of units available was an odd number, so if RCI is double counting, something would have to show up as only available as one or the other.
Sure enough:
1) Attitash Mountain Village, 15 units appear to be available on listing of resorts
When I look at the detailed availability I find:
11 exchange only
2 both
These add up to 15, but in total 13 units available for exchange and 2 available for rent, for a total of 15
2) Bon Temps Village 11 units appear to be available on listing of resorts
When I look at the detailed availability I find:
1 only rent 575.99
6 exchange only
2 both
These add up to 9, but again, total exchanges of 8 plus total rentals of 3 equals the 11 shown on the resort listing.
So I have just now looked at October 2010, because there should be last call availability.
Alhambra shows 27 units available on the resort listing.
The detailed listing shows only 3 units!
all are available for exchange
all are available for rental from $234-$284, so they all count as last call inventory. Each unit is being counted 3 times!
I look at November 2010, because some of those weeks should not be available as last call:
Resort listing shows 30 units available
Drilldown show 11 (which would suggest 33 total available on the resort listing)
Because it only shows 30, I suspect we have 11 exchanges, 11 rentals, and only 8 available as last call. 3 units have a check-in date of november 27, which would be more than 45 days away, and accounts for them not being last call units.
Thus my conclusion is that when searching in this manner, RCI may count the same "unit" (or unit type) up to 3 times when showing the listing of resorts available, though the actual unit count appears an drill-down to the resort level:
Once if the resort is available as an exchange
Once if it is available as a rental
Once if it is available as Last Call
And yes, this seems kind of silly, because if it is a last call unit, it is also obviously a rental unit.
This still doesn't answer the original question of why units seem to be disappearing, but it does solve one mystery of how the online exchange system operates.
My bet on the missing weeks is the behavior of RCI members. This is October, and while some weeks are being taken out of the system, I suspect not as many are being replaced by member deposits. October is not a timeframe when I expect large numbers of deposits. We also may be seeing a change in the way it manages deposits from other sources. When a resort that bulkbanks their excess inventory, in the past we have seen that inventory dumped into the system in blocks, which has resulted in a temporary imbalance between supply and demand. Perhaps they noticed this as they prepare for the new system, and are making a correction.
If I ran their system, I would hold those weeks in a separate pool until such time as a member requested to use one as a deposit. That way those weeks would trickle into the system and create less of an impact.
Unfortunately there are any number of reasons availability may have dropped, and unless RCI decides to share the reason with us, we can only speculate.