Genetics question

Why is genetics weird sometimes? I'm 5'9" with 5'9" and 5'1" parents meanwhile my younger male cousin is 6'0" at 13 with 5'6" and 5'3" parents. How is he so tall with short parents? Our grandparents are like 5'7" and 5'2" and we come from a country where the average is like 5'8 or 5'9" (Korea). I'm genuinely curious on how genetics really works in determining height?