A lot of their claims seem a little fantastical to me, with regards to their tankers and pilots. Given that the Nazis were well known for creating huge amounts of propaganda, it seems that exaggerating the exploits of their soldiers would dovetail nicely with their whole master race thing.
Yet, it seems that their claims are more or less accepted without question. Are they credible, or has no one ever bothered trying to verify them? I can think of at least one situation where the Nazi claims do not match up with American records at all.
Apart from early war aspects of using relatively advanced equipment against inferior and unprepared foes. The Luftwaffe had a very strict service policy negating extended leaves, rotations, and related aspects that the Allies typically employed. This made single pilots flying thousands of missions have ample time to increase their kills. http://acepilots.com/german/ger_aces.html