don't agree entirely with the explanation of why german pilots racked up big scores. At least some of the reason was that certain pilots would be credited with kills made by other LW pilots in the same unit so as to boost the prestige of the unit.
In the 1990s, the German archives made available microfilm rolls of wartime records, not seen since January 1945, available to the public. These showed that while in theory the Luftwaffe did not accept a kill without a witness, which was considered only a probable, in practice some units habitually submitted unwitnessed claims and these quite often were accepted as confirmed. This was particularly true for claims if they were made by pilots with already established records. In theory the Luftwaffe did not accept shared claims, but it happened and often because of the unit citation system, a claim would be credited to the lead scorer, and then also to the person who actually did the shooting as well.
In 1943 the daily OKW communiques of this period habitually overstated American bomber losses by a factor of two times or more. Defenders of German fighter pilots have always maintained that these were reduced during the confirmation process. But the microfilms prove this not to be the case. Some 80 - 90 percent of the claims submitted were confirmed or found to be "in order" for confirmation up to the time the system broke down altogether in 1945.
Another thing worth noting is that the Japanese airforces, I have read, had the most stringent confirmation requirements for claims, whereas the germans on the eastern front had about the worst. Japanese kills had to be confirmed by a pilot from another unit also flying in the area, something not easy to do. german claims could be confirmed by anyone as I understand it.
The reason the Japanese Airforces did not rotate aircrews for training was two fold. The first was that there were simply not the reserves available to allow proper unit rotations. whereas the US entered the war with somewhere between 5-10000 reserve aircrews , the Japanese entered the war with less than 1000. The lower levels of technology related education in Japan made aircrew training a slow process, with a high wash out rate. Later when the Japanese realized the crisis was happening and they began to churn out large numbers of low quality pilots the numbers of experienced aircrew available had dropped even further. there simply weren't enough experienced pilots to spare to go sending them off to the training schools to re-form shattered formations.
This leads into the second problem facing the Japanese. Their garrisoning requirements were so vast, and shipping so scarce that they simply did not have the thruput capacity to afford the luxury of rotating formations correctly. The general movement of shipping for the Japanese was to ship out military supplies and hardware from the home islands, transfer that shipping after offloading back to the resource areas of the far east and then return to japan laden with those resources. this increased the efficiency of shipping use by reducing its 'down" time, but would have meant long delays in unit restoration if shattered units had been transported back to the home islands for training. Whereas the LW shifting of training functions was a wilful and irresponsible misuse of manpower, the Japanese had no choice, they were forced by circumstance to make bad decisions.