In most cases, this is caused by 2 events:
1) the difference in scrotum size. Those with smaller tighter scrotum will not have testicles that descend as low as those with larger scrotums.
2) shorter spermatic cords.
testicles are amazing things, and as every man is aware, they are constantly descending and rising within the scrotum. In warm environments they loosen, in cold they ascend to protect sperm from the cold. Sperm efficacy is greatly affected by temperature, and optimal conditions are slightly lower than body temperature.
This is caused by the cremasteric muscle, which is part of the spermatic cord. The cremastatic muscle tightens in cold conditons, loosens in warm conditions. It also tightens in stressful conditions, an evolutionary event developed to protect the testicles during a fight or attack.
Sexual activity and orgasms also trigger the cremastatic to contract. This happens primarily to protect the testicles from damage during thrusting. In some men, the testicles actually ascend into the groin, and are barely visible.
As men age, their scrotums sag, and their spermatic cords extend. This is caused by both gravity, and loss of elasticity in the cremastatic muscle.