In fairness to Kerry - who I am 100% convinced lied about his actions - it can be very hard to document things years later. I have an Air Medal whose citation has no connection with reality. 1 1/2 years after the paperwork was submitted, I heard the citation read - wrong base, wrong dates, etc - but 1 1/2 years had gone by, I hadn't kept any documentation, and the squadron had been disbanded and another unit moved into our old building.
It wasn't a medal for valor - it was just X sorties flown under certain conditions - and I knew I had done the sorties, so I left it.
Of course, I don't run around claiming to be a hero, either. Damning to Kerry is that other crews operating in the same action have no record in their logbooks of the events Kerry claimed.
Huge difference between not keeping your paperwork, and lying to create the paperwork in the first place!
Is it especially true in the armed services? I have a friend who was in the Army during Vietnam (cryptology, not combat); he took to demanding copies of his immunization records after he got a second tetanus shot months after he'd had one because the records weren't there.