It is my understanding that, in a custom tailored jacket, the tail of the jacket should go no lower than the bottom of the fell of the kilt (the stitched down part of the pleats). This allows for free movement of the pleats without interruption by the jacket. Mass produced jackets shoot for averages when dealing with sizes and shapes, and so it appears that the average man's arm length is somewhat longer than the point where the bottom of the fell typically occurs on most men. So, although they are related, the length of the jacket is set by the length of the fell not the relationship between sleeve and hem.

Me thinks that's the deal,