When using the custom filter, the listeners in a specific week appear somewhat fewer than when adding up the listeners on each individual day within that week. Is this some error?

Not really. This is simply how listeners are calculated. Any listener that was listening to your station before, at and after midnight, will be counted once on both days, but only once within the week. So it may appear as there are "ghost" listeners in individual days, however this is just how the statistics work.