Both white and black are not colors - they are masks the chicken is wearing, under the white or black is the actual color of the chicken - hidden by the black or white genes that are in charge. They're masking genes.
A black chicken could be a partridge colored bird genetically, or anything else.
A white chicken could be blue/quail/ or who knows behind there.
So you take two totally UNKNOWABLE birds and combine them, you can get anything at all dependent on the genes "behind" them.
Unfortunately chicken genetics aren't like crayons - smush blackand white together you don't get Gray. Red and Blue also don't make purple.
It's kind of like grabbing cans out of your pantry without looking at the labels and mixing them together when preparing dinner. The cans of green beans and corn may mix together reasonably well, but how about the green beans mixed with peaches? Or the tomato soup and cherry pie filling? How about pea cobbler?
Crossing white to non-white is like grabbing a can without looking at the label.