I have noticed that ground attack claims are even more inflated than air to air claims. Just 2 examples, during the Falaise Gap battle RAF and USAAF pilots claimed hundreds of tanks and thousands of trucks destroyed yet research on the ground actually seem to have attributed around the low tens of tanks and 2 or 300 hundred trucks knocked out by aircraft. Most vehicles seemed to have abandoned out of fuel rather than destroyed.
I agree that was so, and I think is generally caused by similar factor to what made bomber gunner claims so highly overstated, which is inherent inability of the claimant to properly evaluate the result of the attack. Bomber gunners couldn't follow their targets down, strafers couldn't stop by and evaluate the damage to a truck, and that wasn't as obvious as an airplane hitting the ground or the pilot baling out; and the tendency for more friendlies to be shooting at the same target in case of bomber gunners or ground attack than the case of a fighter shooting at another fighther (though duplication could obviously also factor in that case).
And in case of vehicles, the same a/c were often shooting at the same (perhaps already knocked out or abandoned) targets on different missions, not just the same pilots shooting at the same enemy a/c on a single mission. When the US evaluated NK tank wrecks in UN controlled territory (T-34's) after the NK retreat from SK at the end of the first phase of the Korean War, around 100 T-34's initially attribued to a/c v well over 1000 claims in reports. But they also found that many tanks had been hit by more than one type of weapon, so the attributions of cause were partly guess work. Fliers could seldom tell a dead from a live tank, and it could be playing dead (in fact the NK practice according to tank crew POW's was to abandon the vehicle at the approach of a/c, if possible, and man it again later if it hadn't been destroyed), so it made sense to hit it again if there was no more obviously lucrative target around.
For claims against grounded a/c, it seems to have fallen in two separate categories as illustrated by the following example:
-JNAF claims to have destroyed US a/c on the ground in the initial attacks on Luzon in the Philippines in Dec 1941 were basically correct. Against easily visible real a/c without camouflage or protective revetments devastating losses were claimed and in fact suffered, without great exaggeration by the JNAF.
-JAAF claims to have destroyed remaining US a/c on the ground in the subsequent weeks of the campaign were almost entirely erroneous. They claimed a steady stream of additional a/c destroyed on the ground, a simialr number to the initial Navy attacks, but the US lost very few additional a/c on the ground. The claims were against already destroyed a/c sometimes set back up as dummies, or purpose built dummies, or unsuccessful attacks v real a/c protected by revetments.
On Germans v Soviets in opening weeks of 1941 war, AFAIK Soviet losses of a/c on the ground were very heavy. I don't know the exact accuracy of the German claims in that regard (it's discussed but not really nailed down in Bergdstrom's "Black Cross/Red Star" vol 1) but it apparently wasn't a huge overstatement. AFAIK it somewhat fits the example I gave.
Joe