A I understand it - Gates and Allen were 50/50 originally. Allen was doing something else on the side, so Gates "suggested" they reduce Allen's share to 60/40 or something because Gates would screw any one he could. When Bullmer came along he was from the same mould as Gates and they tried to concoct a deal that reduced Allen's shares even less. Allen overheard the discussion and said FU, I am off.
I believed Allen was the better programmer, but I do not know where I got that from.