An attestation says the reserves existed on one specific morning. An audit says the books hold up over a year. Most stablecoins publish the first and let you assume the second.
On 10 March 2023, Silicon Valley Bank failed with roughly $3.3 billion of USDC's reserves sitting inside it. USDC traded down to about $0.87 over the weekend. Nothing about the reserve reports had been false — the dollars were real, fully disclosed, and independently attested. They were just briefly unreachable. That is the gap between a reserve report and actual safety.
— Educational, not financial advice.