I think it has everything to do with the pattern of the snake combined with the pied gene.
Spieds have coloured heads and not much else, from what I've seen of Piebald Lessers, they have very little pattern if any.
Maybe it's the variance in the "Normal" pattern that determines the amount of white. Much like the contrast in albinos.
The theory backs up the claim that certain "lines" produce a higher percentage of High White Pieds.

-Steven