 The traces in the phase diagrams separate whenever the composition of the vapor differs from the composition of the liquid at the same temperature.Suppose the total composition were 50/50%.You could make this composition using 50% of 50/50% vapor and 50% of 50/50% liquid, but you could also make it from 83.33% of 45/55% vapor and 16.67% of 75%/25% liquid, as well as from many other combinations.