Don't bet your life on it...
Some TAs once posted those systems don't show all availability. I personnaly can not confirm, but seem to remember them saying the on-line booking systems available to the public only show up to 7 or 8 rooms in each category, and not even the vacancies in the room blocks reserved to be sold by agencies that sell a lot of Disney.
For example, All Seas, Dreams, etc. reserve blocks of rooms that they sell, and these rooms would not typically show up (even if they are vacant while you are searching.
If those TAs who posted about this (some time ago) or others can clarify...