When it comes to joint motion, there is a wide range of what is considered "normal." Some people notice their elbows don't straighten all the way, while others have extra motion that allows the elbows to go beyond straight. This is all genetically determined, or based on inherited traits from your parents.
The structures around the elbow, such as ligaments, tendons, and attaching muscles, determine flexibility and motion. How the fibers stretch across and around the joint and how tight or loose the fibers are contribute to the final amount of joint motion.