Irish accent) Because one more and it would be too farty.
They proceed to Chapter 9: The Dark Mark
Because if you added one more it'd be two farty.
Because any more would be too farty.
Two-forty (use an Irish accent)... It's two-farty
Because if there were one more, it'd be two forty. (Too farty) This works much better when spoken out loud. The joke how Irish people pronounce "forty."
Because if there be one moar, it'd be too farty!