The reason why the power limit was changed from 10 to 20 joules is mainly due to political pressure from the "paintball" shooters .
The problem is that the paintball federation was not recognized by the shooting federation and the most of paintball markers was more than 9.9 joules powerfull.
But in France to have this kind of "weapon" it was either to have a shooting license or a hunting license.
None of the two federations (hunting and shooting) did not want to hear about paintball. The only political solution that would put everyone agreed was to change the power limit for air guns. (or to no consider paintball markers as weapons)
It was a way for the French Shooting Federation (which is doing very poorly) to offload the problem.