Surely it is to do with the legal power limit of 12ftlbs energy. For the sake of argument say to achieve near the legal limmit a .22 pellet needs to be propelled at 600 ft/sec and as already stated the .22 pellet is the more efficient. Now to achieve the 12ftlb energy level with the .177 pellet it has to be propelled at nearly 800 ft/sec considerably faster than the .22. That is how I understand it