Kill claims are most accurate when the fighting is over friendly territory, because claims can be checked against ground observers statements, wreckage etc.
So for example, during the BoB, the Luftwaffe awarded 3500 kills, whilst shooting down approx 1000 aircraft. The RAF awarded 2500 kills, whilst shooting down 1700 aircraft.
For much of the war, the Luftwaffe was fighting over friendly territory, so their figures should be, and were, more accurate than their opponents.
However, it's interesting to look at many of the top scoring German aces, and the number of kills they scored away from the possibility of cast iron confirmation.
The Luftwaffe required verification of all kills, but where there was no chance of ground observers or wreckage, the word of the claimaints wingman or other squadron member was accepted. Does anyone really believe that ensures accuracy? It certainly didn't during the Battle of Britain.