The 8th. AF originally counted ground kills but later changed the policy to only aerial victories and victory counts were adjusted, if I recall right.
I can guarantee that many early P-38 pilots lost confirmations because the gun camera was mounted in the nose and the resulting shake from being in such close proximity to the guns rendered the footage useless. Jack Ilfrey probably lost several because of this and so did many other P-38 drivers in the MTO. Jack never quoted me a figure but he said he lost "several" just because of this.
As Randy pointed out, because of differing stories from other pilots in the area I am sure there were others that weren't confirmed. Also add in the "cripple" factor, where one pilot clobbers an enemy aircraft and breaks off the attack because the enemy is obviously going down then another pilot steps in and hits the a/c a handful of times and claims either a full or partial victory. Towards the end of the war, there were many instances of this, especially when Me-262s were hit and pilots from a number of different FGs were involved.
Believe it or not there were also pilots who gave kills away. Art Heiden of the 79th. Fighter Squadron is convinced that both Harold Rau and Cy Wilson (20th. Ftr. Grp. C.O.'s) had confirmable kills and they gave the credits away to wingmen or newly arrived pilots to help boost their scores.
From an interview I did with Capt. Heiden:
Q.: In the past we have talked about the discrepancies in records that have led up to many arguments over many pilots confirmed number of air victories, what is your take on all of this?
AWH: "The whole subject of confirmed victories is a mess. God only knows. As an example, I seriously question victory records, especially (20th. Ftr. Grp. C.O.) Rau, Lt. James Bradshaw and (20th. Ftr. Grp. C.O.)Wilson's. As to the two I got, I really don't have a comment. (Officially Heiden is only credited with a 1/2 vic.)
"I am guessing here, that the Claims Review Office was confronted with the problem of comparing the claims, from day to day, with the intelligence gathering of Luftwaffe combat capabilities. These would differ as much as 100%. Reality had to prevail. I could see Rau & Wilson telling Claims Review to deduct from their claims before those of the line pilots. Also, there was the fact that they were busy leading and distracted from trying to make certain of their gunnery."
Q.: What were the requirements, as you remember them, for a confirmed victory?
AWH: "Victory claims requirements were to have seen the enemy aircraft on fire, or crash, or pilot bailout, or disintegrate. Best to have another witness. A big problem was smoke as an engine at full throttle could lay down lots of smoke. Obliviously, a 'smoker' was generally claimed and that was the problem and others that appeared out of control. Rau and Wilson would not have had time to follow any cripples to confirm that they crashed. At debriefings, a pilot had no real proof other than to let the gun-camera be the witness. Even then, the P-38 gun camera film was so bad that it was next to useless because the camera was mounted in the nose with the guns. On P-38L's the camera was relocated to the external mounts pylon."
Jack Ilfrey was credited with six kills while with the 94th. Ftr. Sqdn. but later on one of the credits was split between him and another pilot. This action is what caused the confusion as to who was the first ace in the P-38, Ilfrey or Virgil Smith. Outside of that all of the remaining kills were confirmed (some sources still cite Ilfrey as having eight) and his final kill was definitely confirmed as he collided with a Me-109 over Germany and the entire squadron saw both planes spiraling out of the air. Jack managed to pull out with about 4 feet of his wing looking like "shredded wheat", the German pilot was not as lucky. Jack managed to bring the plane home too.
I've rambled on long enough, sorry folks, I've also probably muddled the water a bit...

Facts are no one really knows, like Randy said above.