It just searches from the beginning of the string, so for tamias finding neotamias, it’s matching on the scientific name synonym of tamias which is listed under neotamias.
sciurus doesn’t find tamiascirus since it starts in the middle of the string (and there is no scientific name synonym that starts with sciurus under tamiasciurus)
I rolled the exact matching with quotes into this feature request: https://forum.inaturalist.org/t/search-for-and-highlight-only-exact-matches/670
so I’ll close this report