Over here, the DEVONthink 3 Share Menu extension, the DEVONthink extension in the Safari toolbar, and the DEVONthink Sorter when shown as menubar extra all open the same thing: Sorter. Some apps (e.g., Agenda) support the DEVONthink share extension, and it is, again, Sorter that opens when the DEVONthink share extension is clicked.
So “why” doesn’t matter. There are three valid ways to access sorter, and “Share” is one of them. I assume this is the result of conscious programming decisions and not random happenstance.