any combination of these:

- before descending he is adding some rocks into his shirt pocket and water bottles.
- maybe he has a more aero wheels and bike, combined with a better aero tuck
- are there turns and twists on the downhill? perhaps he is better at maintaining speed through the corners.
- perhaps on the crest of the hill he is pedaling to get a "running start".
- You have too much leg hair
- better tires / different air pressure in the tires.
- his wheels are the incorrect size in his bike computer
- he is using a stupid phone app that is always off.