I notice my elbows don't straighten all the way. Some of my friends' elbows seem to bend back past the straight point. Is there something wrong with me?
What you are seeing is what is commonly referred to as "within normal limits." The range of "normal" joint motion is on a slight continuum. When the joint forms a straight we say it's at zero degrees of motion. Slightly more than that in an extended position or slightly less in a flexed position might still be called "normal."
Normal variations in anatomic structure account for these differences. It could be the shape of the bone forming the joint. Or it could be how tight or loose the soft tissue structures are around the joint. The soft tissue includes ligaments, muscles, connective tissue, cartilage, and the joint capsule that surrounds the entire joint.
Your shortened elbow range is most likely normal, especially if your elbow position doesn't keep you from doing things and there's no pain.
Tamara A. Scerpella, MD, et al. Ligamentous Laxity and Non-contact Anterior Cruciate Ligament Tears: A Gender-Based Comparison. In Orthopedics. July 2005. Vol. 28. No. 7. Pp. 656-660.