From Apple's technical summary: "The threshold is selected to provide an extremely low (1 in 1 trillion) probability of incorrectly flagging a given account. T

From Apple's technical summary: "The threshold is selected to provide an extremely low (1 in 1 trillion) probability of incorrectly flagging a given account. This is further mitigated by a manual review process wherein Apple reviews each report to confirm there is a match..." So it's 1 in 1 trillion per account PRIOR to manual review in which the odds of error get reduced even further.