How does basketball star jeremy lin get to be 6' 3" when reporters say both his parents are 5' 6"? Just luck? Is it rare to happen?

Growth potential. Genetics essentially provide the potential for height. Environment can determine how much height is actually achieved. It is common for children of immigrants to grow taller -and fatter- than parents in the US due to diet and health reasons... just like American children being taller than American-born grandparents.
Genetics. There were tall people in jeremy's ancestry, whose genes were passed on to him. This happens frequently.