1 in a trillion is not particularly impressive. Aren't there a billion iPhones? And probably 10,000 pictures on each phone on average (people take lots of selfies).
Well, since Talking About Statistics Is Hard, you have to look at the exact phrasing from the website: it "ensures less than a one in one trillion chance per year of incorrectly flagging a given account". So, each of those billion accounts has a 1:1T chance of false positive. If I remember my stat 101 correctly, that should translate to a 1:1000 chance of having at least 1 false positive on the planet during any given year. (And remember, even a false positive just means that a human reviews your photos, not that you get reported to the police.)
