Technically, the white and amount of it are variable. Though my pied girl is low white, from low white parents and the entire clutch was very low white. One of the males had almost no white crossover and the highest white was about 40% white.

I think the thing is: not too many people breed pieds to pieds since it is so much more exciting to breed co-dom to pieds for pastel/spider/whatever hets. Though personally, if i had enough pieds I would probably put them together at least once for an all pied clutch.

So I think it's possibly genetic, but pieds are so variable on their own that breeding medium white pieds together could theoretically result in insanely-white pieds and low whites in the same clutch!