There's a general principal involved, that once you start throwing round numbers like 'one in a million' around, you're almost certainly overlooking larger sources of error. Eg, someone fucking up in the lab is much more likely than one in a million. There was a famous case where they were looking for (what they thought was) a serial killer who showed up in a bunch of DNA tests, and it turned out to be some factory worker who had contaminated the swabs.