i think the deal is that the date range parameters here are actually operating based on datetime rather than date.
compare
https://api.inaturalist.org/v1/identifications?d1=2021-01-30&d2=2021-01-30T23:59
and
https://api.inaturalist.org/v1/identifications?d1=2021-01-30&d2=2021-01-30
i noted something similar on another endpoint, i think: https://forum.inaturalist.org/t/1m-observations-this-month/2582/17.