Modern SharePoint allows you to add users via Sharing, which doesn't add users to the old "advanced" permission groups. Then I noticed that in the "advanced permission" groups, that there is a group within a group. I can't look at the people inside of the second "Human Resources Members" group but I suspect that it is a list of the people with whom the site has been shared. Can someone confirm that? What would happen if I deleted the second, nested "Human Resources Members"?
You made a good point so I checked. There are only two human resource groups I can find.
But if you look at the names of the groups in question, they are worded Members after the type of SharePoint permissions they have. I still thing the nested groups were made by SharePoint when it created the site for me.