Why the children in a family may have different genotype and phenotype than predicted by the punnet squre

Answer: Probability/mutations

Explanation: A punnett square predicts the probability that a child or the offspring will receive specific genes or look a certain way. The genes with the highest percentage or usually the ones that show, but sometimes recessive genes are the ones that show through, which may be what you mean by a different genotype/phenotype being different that what was predicted by the punnett square.

a mutation may also occur that messes up all of this.