The default plugin looks for siblings at the editor, e.g. buttons for new threads or in the editor toolbar. The sibling is always a button, but the PUP creates a DIV container, so it looks a little bit creepy.
If the button is in the toolbar, the PUP creates a DIV container. But all other buttons in the toolbar are dircetly <button></button> container. So the div made some different margins between all other buttons.