You likely have a doc in a workspace and in a folder outside of your “My Docs” folder…
MyDoc Folder
The MyDoc folder is the special folder that you get when you are added to a workspace. This is a private doclist for that others in the workspace cannot see. If you explicitly share a doc from the MyDoc folder then that person will now appear in the dropdown list.Folder Membership
If the doc is in any other workspace folder then the people list comes from the members that have joined the folder. You will also see people that have been explicitly added to the doc as guests through the sharing dialog.Workspace Membership
When adding a doc to the workspace there is a “show more” option in the drop down. This will enable you to browse through all the workspace members and select a person that hasn’t joined the folder yet or been shared the doc.