Ooh! Forget going to bed at a reasonable hour. I was just reading a book that goes over this exact topic. I can only speak to the bombing of Germany and the fighter defense thereof, really, but maybe someone else can add what I miss or talk about the other fronts.

Part of this answer has to do with German fighter doctrine. The British Royal Air Force (1940 onwards) and the American Eighth Air Force (late 1942 onwards) were pretty consistently bombing German cities and factories throughout the war, without much of a break; the RAF bombed overnight, the USAAF during the day. As a reader of Catch-22 would know, the Allied forces would rotate their bomber crews on 20, 25 or 30-mission tours (unlike Heller, the number of missions didn't arbitrarily go up to like 70 so Colonel Cathcart could prance for a general). German pilots fighting in defense of the Reich didn't really do this. They got shot down, they went straight back up again. Many of the Luftwaffe's best and most experienced pilots stayed upright that way. So they might have had more opportunities to accumulate kills, based on flying time and Allied bombers showing up every day and night, than Allied pilots.

Reason #2 has to do specifically with night bombing. There was very much a technological and doctrinal chess game going between the Allies and the Germans, regarding how to bomb on one side and how to shoot bombers down on the other. The British, doing night bombing, eventually figured that for reasons of safety and getting everyone to bomb in the same general area, they would have all the bombers go as much as possible in one long stream, the "bomber stream". One of the German doctrinal innovations for night fighting, eventually, was for each individual fighter aircraft to patrol a three-dimensional box of an area, x by x by x km, called Raüme. Radar beams from the ground would vector you to bombers coming into your box, and you would kill them.

Now, if the bombers are all going in one long stream, they're only going to go through so many boxes at once. Those would be the fighters that would get kills. Over time, as the Germans got new technology, this would change, and fighters would be allowed to attack the bomber stream at will. There was a nasty little cannon called Schrage Musik, or jazz music, that was implanted in the German night fighters and fired directly upwards. Because the British bombers had a blind spot underneath them (no rotating "ball" turret underneath like the American B-17 or B-24), it was very hard for them to detect the Schrage Musik-equipped fighters from easing up underneath undetected and killing them. This is again only part of the answer, but it's one weapon that German pilots could use with great effectiveness against the bombers, and helped them rack up kills.

Reason #3 was US bomber doctrine. The British and the US prewar doctrine involved getting a huge force of medium and heavy bombers, flying over their enemies and bombing them directly out of the war (i.e. enemy industrial capacity would be devastated and the people's morale would collapse). Well, the British figured out very quickly that even if you slapped a lot of machine guns onto a bomber, if you sent it out in daylight without a fighter escort, you were distressingly likely to lose it. German ME-109 and FW-190 fighters, based on their heavy firepower, superior maneuverability, tactics (like attacking right at the nose, dead-on, where bombers had only minimal machine-gun fire ability early in the war) and experience, were very good at killing unescorted bombers in daylight. The British figured this out pretty early and switched almost completely to night bombing, where the bombers were less vulnerable. The Americans, when they arrived, took longer--B-17s and B-24s were more heavily armed than the British Lancaster or Stirling, and they figured that by assembling enough bombers into a formation, it would become impregnable to enemy fighters. That did not happen, and the USAAF lost a LOT of bombers to figure this out, before the P-51 Mustang came into service with drop tanks attached and they were finally able to get fighter escorts all the way over Germany.

Those are some reasons why bomber-killing pilots were able to rack up high scores fighting the Western Allies' air forces in the defensive battle over Germany. However, I picked some random pilots from the list, and several of them--Wilhelm Lemke, Heinrich Setz, Gerhard Thyben--list astronomical kill totals over the Eastern Front. Maybe someone who's expert on the Soviet air force can shed some light here? (Possible reason: The No. 1 ace of all time, Erich Hartmann, reveals that early Soviet aircraft didn't actually have gunsights--they just painted them on their windshields! If that is any indication of their overall training or their airplanes' capabilities, no wonder the Luftwaffe dominated!)

Source: Robin Neillands, The Bomber War: The Allied Air Offensive Against Nazi Germany.