Please have a look at those posts. It depends on where you insert the citation. If your cursor is on an existing citation all new references you add will be added to this existing citation. If your cursor is next to an existing citation a new citation will be created.
For technical reasons, Paperpile cannot be smarter about that because at the time you start inserting a citation we don’t know what’s left or right to your cursor, we only know what’s on the exact cursor position.