Im skinny, but nearly overweight?

This confuses me... I'm male, 5ft 7in, and weight 10st 10lbs. This puts my BMI at 24, which is right on the border of being overweight.... Yet I'm the skinniest person I know... I wear Small or X-Small tshirts and size 32 trousers.... Why would my bmi be so high when I'm so small? I thought I was a good weight...