ChickieMamma- Cute ducklings! Henk already gave you a spot on explanation of how you got that color in your other thread. I don't want to go into a lengthy explanation on the board, but can give you more info. via PM if you need more, but solid White in domestic ducks is virtually never "pure" in the sense that a lot of people
seem to think of the term "pure". There is virtually
always something behind the white genetically. In breeding tests, when breeders have crossed solid White to Grey/Mallard (typically used for breeding tests), getting Black Bibbed is extremely common, which basically means that a lot of Pekins (and hence "White Cresteds") carry those genes behind the white. Your drake also obviously carries sex-linked brown dilution, which combined with the extended Black to give Chocolate.
What is unusual is that you bred so many before a Chocolate popped up. It is certainly not unheard of though. When we say that any given breeding will give 25% of this or 50% of that, this is only an average percentage over a large number of matings. It doesn't necessarily mean that if you breed two offspring for example 50% will be one thing and 50% another. Granted, as I said, it is unusual that it took so long to pop up, but it can happen. Good luck with your breeding! Cute babies.