You seem to have 3 different minimum densities there. One is the edge, which should be base + fog only, one is the center unexposed area which also should be base + fog, and the third is the pair of lines which are in question.

So, how come the b+f is so high, and has 2 values, one on the edge and one in the unexposed center area? If we can explain that, maybe the two edge lines can become more understandable.

Can you shed some light on this?