7% oleic acid is not zero oleic acid, right? And what looks like "a lot" to the eye is not necessarily a lot by weight.
And I called it oleic soap for short, because that is the major unsaturated fat in most soaps. But any of the other unsaturated fatty acids in your soap will also be concentrated in the yellow portions.
edit -- Another factor is the water content may also vary between the different regions. Hard to say for sure, but there's a crystallization process going on if what I think is happening is indeed happening, and that will affect the location of the water as well as the location and appearance of the different types of pure soap.