The lens lineup (optical design) and optical optimization is usually different among these cases. f:1,4 "normal" lenses are designed for best performance at wider apertures, where usually the f:1,8 gets "weak". But on the other side, the cheaper f:1,8 usually beats the more expensive f:1,4 in sharpness with diaphragm fully closed.

You see, as in most cases, one's strong point is the other one's weak.