Pierre,
As I am sure you already know, there is no magic about sailing. If the guy is sailing away from you downwind he is doing it because he is doing the basics better.
A good hoist will save you 5 seconds on a bad one.
Same for a drop
Same for a gybe
Thats 15 seconds you have lost
Very small rudder movements will give you some more seconds on the course.
Indecision will cost more seconds.
A dity hull will cost more seconds.
A cleat that doesnt jam properly will cost seconds
The seconds add up to your competitor getting away every time.
I used race Solings for a number of seasons, and was lucky to be on one of the "faster" ones. A few times a year we did sprint races. Probably 500m to the top mark, 1 round. Starts were critical, picking the 1 or 2 shifts and gusts was critical etc. Often the antifouled boats could beat us if they did teh basics right. We may have had the faster hull but it made no difference on a course where tiny errors were punished. It was close to the best racing I have ever done.

If you are sure that the boatskills are equivalent then he must have a lighter boat (or a bigger spinny), or the engine on ;-)
JB