Given that tens of thousands of people flew a fighter in the Second World War, I'd be surprised if there weren't a few frauds (and a few saints) in that number - every kind of person, in fact. But you just don't
need deliberate fraud to explain most overclaiming. You could never prove it but after a career working in offices I wouldn't be surprised if the difference between JG 2 and JG 26 wasn't simply how each organisation interpreted and applied the rules before submitting a claim to the
Abschusskommission.
I've posted
research on I./JG 2's combat record in Italy (again about a 2:1 claim/kill ratio) but the thing I found hardest to account for was the unit's gaining a hotshot reputation with both sides within days of entering combat, which its overall results don't necessarily support.
A final point: even if (say) Hartmann "only" shot down 70 or 80, that is a spectacular performance when you recognise that most pilots never shot down any.