Thedibb has (or used to have) a calendar that shows you open fastpasses. To keep from completely overwhelming the Disney system, these third party sites will limit variables (like party size) when doing these checks.
If touringplans, thedibb, etc, called every party size available, then the system wouldn't be able to actually process FPs for people in the parks. Also, the people in charge of the FP system might block repeated inquiries, which would result in us getting no information, at all.