Why is it that for a certain date, WDW reservations has multiple accomodations open at the dvc resorts but DVC member services has none. How does that work exactly?
Disney CRO holds something like 4% of the inventory of rooms at each DVC resort. DVC member services holds their own separate inventory. So they are 2 totally separate inventories Whenever a member trades out for accommodations other than the DVC resorts (i.e. - stays on pts at the GF,POLY,cruiseline,etc.) those get turned over to CRO in the trade and become part of their inventory as well.
Disney retains ownership of a certain percentage of each resort (e.g., 4% at BWV). Disney also gets a slate of rental opportunities whenever a member trades out of DVC for another resort (at WDW or elsewhere). Those rentals thus controlled by Disney are not available to DVC members and may be available for rent to the general public even though all the remaining room/times have been filled by DVC reservations.
Sometimes if you call last minute they can work some "behind the scenes magic" to open up some empty rooms for points. I assume they trade off with some other dates.