IUCN considers species only on a global or continental scale, while NatureServe considers species globally (global ranks, or g-ranks) and locally (state ranks, or s-ranks). So G1 is similar to CR, a G2 similar to EN, etc. In North America, many plant* species don’t have an assigned IUCN ranking, but most have a global NatureServe ranking (even if they are secure/G5), and species will also have state ranks (at least in states where the species is considered to be rare, threatened, or endangered- S3, S2, or S1). A species can be globally secure (G5)/least concern (LC) but locally rare (S3), and therefore have a “worse” state rank. So oftentimes a species state rank is what dictates what shows up in iNat Threatened results.
*I’m referencing plants here because I’m a botanist; IUCN has more ranks for other taxa like fungi, animals, etc.
However, the opposite can sometimes happen too. For example, Eastern hemlock (Tsuga canadensis) is still reasonably widespread and common in the eastern United States. However, invasive insects (including the hemlock wooly adelgid) are decimating populations across its range and the species is rapidly declining. Because of this, IUCN has therefore given it a global rank of NT (near threatened), and NatureServe has a global status of G4 (apparently secure)- these are reasonably equivalent. However, many states haven’t yet updated their statuses because they still have a lot of healthy hemlock (in Pennsylvania it is still S5/secure). Therefore the IUCN status is “worse” and that’s what gets used for the iNat Threatened list- hence why it is the most common plant species that shows up when filtering for Threatened.
As far as I know, any status other than G5/S5/LC can result in a species showing up under Threatened. This also includes “sensitive” taxa that have been given that status due to threat of overharvesting. For example, wild yam (Dioscorea villosa) is common/widespread species (LC/least concern and G4G5/apparently secure) but has been given a status of At Risk by United Plant Savers because of “concerns of overcollection for medicinal use.” That status is enough to override every other local/global status suggesting that it is common or secure.
I’m not sure if that helps clear up any confusion. Perhaps if you mentioned which species you suspect are showing up erroneously under Threatened we could figure out what’s going on?
(doing my best to bite my tongue about the lack of oversight or standardization when it comes to conservation ranks on iNat, which seems to result in errors and confusion- but that’s an argument for another day).