this seems to be it (and i think more specifically, it’s probably more expensive to sort by species count than by observation count), but i don’t think staff have ever confirmed that this is the intent. more information here: https://forum.inaturalist.org/t/cutoff-for-2023-top-500-exceeds-6-000-observations/47901/27.
correction: i think staff have said this is intentional: https://forum.inaturalist.org/t/incorrect-calculation-for-most-species-on-the-project-page/62501/4.