Well...your question is the answer. Since not every child of the same parents are the same, the eggs and sperm must be different.
How children turn out is based on heredity, which means that we inherit certain traits from our parents. Each egg and each sperm are not only different, but they also respond differently to temperature, stress, time of day and so on. So the entire process from the creation of the egg/sperm cells to the conception plays a part in which traits will be passed on to the child.
Here's a page:
Genetics
Tormod
