Luftwaffepersonnel policy - The Germans tended to keep pilots in combat for as long as possible. In general, German jadgflieger only stopped flying combat missions if they were killed (Walter Nowotny), badly wounded (Günther Specht, although he eventually returned to service), captured (Ulrich Steinhilper), or promoted (Adolf Galland). By contrast, leading Allied pilots were often promoted to non-flying jobs leading operational units, put on staff duties, or sent to help train up new pilots. Battle of Britain ace "Sailor" Malan became Biggin Hill's station commander in 1942, which limited his ability to fly on operations. U.S. Navy ace "Jimmy" Thach, the inventor of the famous "Thach Weave" maneuver, was transferred to training and staff duties after he'd scored only six kills.
Length of time in combat - Virtually every high-scoring German ace flew their first missions over Poland in 1939 or France and Belgium in 1940. Some pilots, like Adolf Galland (96 kills) and Werner Mölders (102-108 kills) had flown with the Condor Legion in the 1930s. Mölders, for example, already had valuable combat experience and several kill claims from the Spanish Civil War before WWII even began. By contrast, Soviet pilots didn't enter combat until June 1941. It wasn't until 1943 that USAAF operations in the ETO began to kick into high gear. And it wasn't until mid-1942 that the USAAF and USN were able to really contest the Japanese in earnest. In short, German pilots had a head start over the pilots of nearly every other Allied nation, with the exception of the RAF. According to Trevor Constable and Raymond Toliver: "One American pilot with 254 missions actually fired his guns at an airborne target only 83 times, and this may well be the best record of any American pilot. In gaining his 352 aerial victories, which establish him as the leading fighter ace of the world and of all time, Major Erich Hartmann flew 1,425 missions. Many times Hartmann flew from two to seven missions a day ... He was involved in more than 800 actual aerial combats." They go on to point out that the average German pilot flew around 1,000 to 2,000 combat missions. Meanwhile, the most active Allied fighter pilots flew only from 250 to 400 sorties.
Quality of the opposition - I don't want to give an unfair impression of the Soviet Air Forces, but I think it's fair to say they weren't ready for war in 1941. The VVS operated a large number of obsolete aircraft like the I-153 biplane and the I-16, with its open-cockpit. Other early-war aircraft like the LaGG-3 also had serious issues (poor build quality, underpowered, etc.). The Germans were able to destroy nearly 4,000 Soviet planes in the air and on the ground during the opening week of Operation Barbarossa. By 1943, the Soviet Air Forces were a much more formidable opponent and by late 1944, they could serious contest air superiority on the Eastern Front. But in 1941-1942, German pilots had a real chance to run up the score against the Soviets. You can make a similar case that the Gloster Gladiators and Hawker Hurricanes of the RAF offered similarly-vulnerable targets for German pilots in North Africa during 1941.
Number of opportunities - By 1943 and 1944, the Luftwaffe was getting increasingly outnumbered in the skies. That meant relatively few chances for Allied pilots to get kills and that those kills would get spread out over a larger number of pilots. It also meant the reverse for the top German pilots, who had a target-rich environment with lots of chances to get kills. Gunther Rall mentions this in his veryz-enlightening interview here: https://m.youtube.com/watch?v=STFdRrWBW2w&t=1m0s
Lying,over-claiming, and wishful thinking - I don't want to single German pilots out too heavily for this one, since aircrew on both sides often wildly-inflated kill claims. Part of this is a product of the confusion and stress of combat. By comparing kill claims to actual loss records, it's cleae pilots often counted damaged planes as "destroyed" or double-counted another pilot's kill. For example, on September 15th, 1941, JG27 claimed 20 kills against RAF and RAAF aircraft over North Africa. However, Christopher Shore, who has written extensively on the air war in the desert, has found that Allied records only report 5 losses on that date. In other words, JG27 had a 3:1 bogus claim to real claim ratio. This is one reason why gun camera footage of a plane exploding or a pilot bailing out was so helpful in verifying a kill claim.
German "one pilot, one kill" accounting rules - Luftwaffe kill counting gave only one pilot credit for each destroyed aircraft. It's not hard to imagine how this "one pilot, one kill" system advantaged higher-ranked veteran pilots over more junior fliers. By contrast, the Commonwealth and American kill systems allowed for fractional kills credits. Credit for one victory could be split between multiple pilots. This is why RAAF ace Cive Caldwell has 28.5 victories and Mustang ace George Preddy has the even-odder score of 26.83 victories.
American claims given more post-war official scrutiny. - In the 1950s, American kill claims were evaluated by the USAF's Fighter Victory Credits Board. This Board used information only available after the war (ex. German loss records) to reevaluate American wartime kill claims. This was followed by another board in 1978. In 1945, Mustang ace George Preddy had 27.5 kills. After the 1950s Board, he was credited with only 25.83 kills. And after the 1978 Board, he had 26.83 kills. To my knowledge, the Bundswehr has never subjected WWII German kill claims to the same level of post-war scrutiny.