So closer to 1/10M. The reporting threshold is made artificially higher by requiring more than one positive.

But anyway, that's beside the point.

A perceptual hash is not uniformly distributed; it's not a random number. Likewise for photos taken in a specific setting; they do not approach the randomness of a set of random images.

So someone snapping a photos in a setting that has features similar to a set of photos in the CSAM database may risk a massively higher false positive rate. It's no longer a million sided dice, it could be a thousand sided dice when your outputs happen to be clustered around similar values due to similar setting.

But I can't say I care about false positives. To me the system is bad either way.