There are a number of things that explain the discrepancy.
First, the list of 10 is a checklist. Checklists are known to be a little buggy and are not recommended (and will be phased out at some point), plus they don’t include casual observations.
If you instead use Explore to list mammals on the island, and you allow for casual observations (because your list of 32 allows casual), then you get 20 instead of 10.
So what is missing between the 20 and the 32 from the drawn circle? It’s not just “a few sea mammal species”, the missing 12 species are 11 sea mammals and the bat that you mentioned. The sea mammals are easily explained by the difference in the shape between the two places, and the bat is missing because it is threatened and the locations are obscured, more about geoprivacy here.
If you want to include near shore observations and obscured observations, I would recommend using the standard (“official”) places for the municipalities on the island. You can string them all together in the URL and then bookmark it, or you can make a project that uses them.
https://www.inaturalist.org/observations?place_id=48888,48889,48892,48890,48886&verifiable=any&iconic_taxa=Mammalia
