The claims shown in Hartman's book (written many years ago) as well as records from III./JG 52 were never compared to Soviet losses until those records became available when the Soviet Union fell, that's where these "sudden" Soviet documents came from. As mentioned earlier, the losses shown in these records had no indication on who brought down the listed aircraft. The people who researched Hartman (as well as other aces) were just as surprised to see this disparity as they were to see the accuracy of other aces researched. If the 44% inaccuracy was applied to all of Hartman's claims, it would show he had 197.2 kills. At 20% as you mention, it would put him at 281. Bottom line, it's clear that Hartman overclaimed substantially, Now we can discuss the reasons behind this but considering what Hartman went through during and after the war, I don't believe there was any malicious intent.
1.Hartmann like all other pilots and the Luftwaffe fighter-Aces was very well aware as to how kills were awarded.
2.Hartmann off-course was aware about the immense pressure from Lw HQ. onto the respective Wing-commanders - down to group Commanders - down to Squadron-leaders
to report kills and produce Aces.
As such many kills from other pilots were attributed to the Aces or the most likely pilot of a unit to reach e.g. 25/50/75/100/150 etc. and favored by the propaganda apparatus.
Another issue was that the wing-man or other new-bees were "miss"- used to provide distraction for the enemy so that the Ace could insure his rising kill claims.
In regards to over-claiming: e.g. 4 Luftwaffe pilots after their mission, claimed a total of 6 enemy aircraft. I pilot claimed 4 and the remaining three pilots claimed 2 - however e.g. the RAF or Soviet stats record only 1 aircraft lost and 1 returning very badly damaged. Usually because pilots were instructed taught, reminded - swoop in shoot and turn away, do not loiter around to watch the enemy aircraft hitting the ground or pilot bailing out. So in that case how would the respective pilot know that the enemy aircraft that he hit (flying debris/smoke) actually did crash and not continue to land somewhere.
E.g. those film cameras showing a B-17 being hit all over the place - but (at least me) I have not seen the continuation showing that specific B-17 to actually crash. Especially bomber wrecks were partially easy to be found on German ground - either the pilot who had hit this aircraft had memorized a number or distinction that fitted the wreck - bingo, but even that did not exclude that a badly damaged B-17 was then attacked by e.g. 2 other pilots - so initially 3 pilots claiming a bomber each. The propaganda and the wing commander were both rather interested to report 3 Anglo-American Terror-bombers downed then 1. The funny part is however that in truth that B-17 had already been crippled beyond repair by a Flak unit - before the fighters even came.
Now we got 3 claims by the Luftwaffe fighters and 1 by the Flak. so 4 for actually only 1 B-17 lost.
And then you got the cheaters/liars who simply claimed an aircraft - supposedly e.g. Franz von Werra was such a pilot already before his POW and escape history. After he came back
he became a Hitler and propaganda all-time favorite - so which superior would deny him some kills - even if they sounded doubtful? and again supposedly there were quite a few
of these pilot Aces being "slandered" with such comments.
I remember in this book regarding Marseilles there is an episode were after he had landed and claimed (can't remember) e.g. 8 aircraft's in a single mission and the mechanic inspected the weapons reported that M. had only fired (can't remember) 50 shots. In the book this was used to explain his mastery in aiming skills. so ???
Regards
Jagdflieger