So the way we think about these things these days, the difference between A & C is that C simply corresponds to a larger amount of fibres, with an otherwise identical spread in the orientations. B on the other hand also corresponds to a larger amount of fibres than A, but with a greater spread in the orientations (the apparent fibre density in B & C might actually be similar, but it’s hard to tell). This is because the amount of fibre (the apparent fibre density) is essentially the integral of the FOD over the solid angle corresponding to the fibre bundle of interest. So the FOD B & C might have a similar amount of fibres overall (i.e. similar FOD integrals), but because there is greater dispersion in B, its peak amplitude is by necessity lower.
Importantly, the fact that A & B have similar peak amplitudes is essentially meaningless given the difference in dispersion - at least if what you’re interested in is the apparent fibre density.
Does this answer your question…?