The U.S. (not sure about the RAF) brought ALL of its experienced pilots home after they completed a tour (25, later 50, missions) and used them as training cadre. This allowed the experiences of combat to be taught to the next generation of combat pilot.
None of the RAF or U.S. pilots were ever going to get close to the German aces. They, especially the Americans, only had one chance to get shot down and it was "for you, the war IS over!" (there was at least one case, probably many more, of a German pilot getting shot down in the morning, catching a ride back to base, and taking a different fighter up that same afternoon) and limited chances for combat. German pilots often flew more than one combat mission a day, while allied pilots might fly 10-15 missions a month once the BoB was over.
The allied pilots also were fighting against other fighters, not against bombers. It took tons of guts to attack the bomber boxes, but the bombers, before they had escorts, were a far easier kill than a single engine fighter, especially once the American/RAF pilots got a bit of experience.
Lastly, the German pilots (and to an extent, the Japanese) got lots of kills against obsolete or obsolecent designs, especially on the Eastern Front or in China, early in the war. The American & British pilots had far fewer opprotunities to fatten up on biplanes and outdated transports flown by ill trained pilots (although the USN & USAAF pilots had more of a chance of this against the Japanese as the war entered 1944).
It is noteworthy that the top three American scores were rung up against the Japanese, where American pilots had the chance to go into combat against bombers and obsolecent fighters as the war ran on. Even in the Pacific the American pilots still lacked many of the "advantages" of their German counterparts since they had far fewer chances to engage in combat (Carrier strikes were far from an everyday event) and getting shot down was, as often as not, a one way trip to a watery grave, with POW status being the next most likely.
Given all the differences between the Allied and Axis pilots opprotunities it is almost beyond imagination that ANY allied pilot would get to Hartmann's 352 victories (1400+ missions, 345 victories on the Eastern Front, shot or forced down at least a dozen times) or even Galland's 103 victories (Galland was shot down at least THREE times).