Anytime a white bird has been used anywhere back in a bird's history, you have a chance of throwing whites if 2 birds that carry that recessive gene are paired. All the recessive white is an 'off' switch for color. A white bird still carries whatever color, but it just shuts off the color and makes the bird white. For example, 2 years ago we had a couple white ducklings pop up. We only have exhibition rouens and black/blue muscovies, not a white bird on the place. I let them grow out and they turned out like perfectly white rouens. I talked to a few old breeders and they said many years ago, some breeders crossed in Ayelsburies to improve the type on the rouens. I may have just gotten the luck of the draw and had 2 birds that remotely carried that gene to cross. My mom also bought a quad of black and blues on that standard cochins. Had a white pullet pop up there too this year....
You also have to think that white was the original color on many breeds. Take silkies for example... White has been in the standard for over 100 years. Black has been in since about 1960 and bearded splash and buff as recent as 2002. They had to cross in other breeds somewhere back there to introduce that color in silkies, but it all started with the whites. A lot of breeders today even cross colored birds onto whites to improve type and feathering.