that’s because different types of place are involved here. The reason that Sigüenza appears as the rectangle for you is that you’re searching for that name via the Google Maps search, and that’s how it retrieves/defines that place. For Madrid, however, an exact polygon exists on iNat, thus why you see those accurate borders. This is because that place is a standard place, the shapefile for which was imported from GADM.
if you would like Sigüenza to appear as an exact polygon, you can create a community curated place for it
see https://forum.inaturalist.org/t/use-openstreetmap-maps/2588 for a discussion of Openstreetmap