The white silkies most commonly are is recessive white. Meaning that for it to make an appreaane (or technically a lack of appearance, lol), the bird must receive a copy from each parent. Chances are reasonable that there are many non-white silkies in pure coloured flocks that have not been bred to white for many generations that carry a hidden recessive white gene. If two of these white carriers pair up, 25% of their offspring will be white.
Now say you have a flock of 10 hens and 1 cock, and the hens all lay daily. Say the cock and one hen carry recesive white. You gather eggs for 8 days (makes the math simplier than a week) and hatch the 80 eggs. Theoretically, 2 of the 80 eggs should hatch white chicks. However, consider that the hatch rate is not likely to be 100%. If the hatch rate is 85%, 12 of the eggs will not hatch; who's to say which 12, and whether those two are in it? Lower the hatch rate further, and the number of eggs that do not hatch increases. If you just consider the 8 eggs laid by the hen carrying recessive white, 1.2 will not hatch. Just considering the 2 recessive white eggs, .3 will not hatch. Obviously, .2 or .3 eggs doesn't "work" but it shows the odds.
And in any case, of the 8 eggs laid by the recessive white carrier, while two will carry recessive white from both parents, another 2 will inherit it only from the papa & another 2 from only the mama. But you will never know that those 4 carry it until THEY are paired with a bird who also carrries it.