The difference between Accipter and Egretta is that for the genus Accipter, ‘Accipters’ is the common name, but for the genus Egretta, there is no common name.
It’s tricky because sometimes the common name IS the scientific name, and other times it is not. From your list again, Anas is also genus, and its common name is ‘Mallards, Pintails, and Allies’ not ‘Anas’. If ‘Egrettas’ was a commonly used way to refer to species in the genus Egretta, it would be a common name. But there is no current common name used to refer to all the species in the genus Egretta, just the scientific name for the genus. You and I and everyone else could start referring to them as ‘Medium-sized herons’ and eventually, that could be a commonly accepted common name for the species in the genus Egretta. Or we could try ‘Egrettas’ but neither of those are currently the common name for that genus, and there is no current common name for that genus in English.
I guess what I’m getting at is:
It’s not an inconsistency with the iNaturalist data explorer. It’s the nature of common names that sometimes there are good and helpful and unique identifiers, and other times there are not (see this thread here for how duplicated common names sometimes murky things up in the identification process: https://forum.inaturalist.org/t/what-things-are-misidentified-as-large-milkweed-bug/12571/16?u=alexis18). That’s why we have scientific names - a unique identifier that has been described and typed so that when we discuss a species we are all in agreement about what is being talked about and described. But a common name is often not unique, not consistent across the range, or may not exist at all.