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."
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Two-thirty-nine. One more and it'd be too farty.
Irish accent) Because one more and it would be too farty.
Because if there be one moar, it'd be too farty!
Because if you add even one more it gets "2 farty".