There are two terms in genetics that will explain this. Phenotype is what the parent looks like on the outside. Genotype is the genes that the parent is carrying, whether visible on the outside or hidden on the inside. Without considering any modifying factors, the dominant genes in the genotype determine what the parent looks like, the recessive genes are hidden, but can contribute equally to the looks of the offspring. Also, some appearances require a mix of genes, some from one parent and some from another.
When you breed standard bred birds, you can get a predictable outcome. All the chicks may not look alike, but if you breed enough of them you can predict what percentage will be one color and what will be another color, etc. When you breed birds that are not standard bred, so you only know what's in the phenotype but not what's hidden in their genotype, the resulting chicks are completely unpredictable. There is usually a huge variation in colors and sizes and other features, often times looking nothing like the parents at all.