I think BA's aim is to give you every destination available in one (or more months). AA will give you just a single day, but of course that is a smaller result and looks less impressive to a potential customer.

It's obviously possible to live-check each destination live for a given day, as that's what AA do, but searching each destination for each day of the month or year would presumably take too long. So instead, it would appear, BA have a back-end static list of when each destination is available. This can be pulled on to the map more quickly, but it can't always be up to date.

The problem is it seems to always be more than a little inaccurate.