They exploit the fact that so much of our DNA is the same. They basically have the book with no typos, or rather with only the typos they've decided to call can

They exploit the fact that so much of our DNA is the same. They basically have the book with no typos, or rather with only the typos they've decided to call canonical. So given a short sentence excerpt, even with a few errors thrown in, partial string matching is usually able to figure out where in the book it was likely from. Sometimes there may be more possibilities, but then you can look at overlaps and count how many times a particular variant appears in one context vs. another. One problem is, DNA contains a lot of copies and repetitive stretches, as if the book had "all work and no play makes jack a dull boy" repeated end to end for a couple of pages. Then it can hard to place where the variant actually is. Longer reads helps with this.