The "algorithm" isn't some sort of neural network trying to intelligently identify things that "look like" CP. It's a perceptual hash matching against a database of known CP. It has to find multiple matches before it flags the account for review to reduce false positives. Only after review confirming a match to known exploitative images is the info referred to NCMEC for action.

Full whitepaper: https://www.apple.com/child-safety/pdf/CSAM_Detection_Techni...