I calculated "63% after 20,000 rolls" exactly from 1/20,000 probability It's how probabilities work: if probability of an event is 1/n, and you perform n trials (and n is a big number), usually you'll get ~ 63% probability of getting an event at least once, because 1 - 1/e ~ 0.63 (if you would do the math, it's easy to show this is the limit when n->infinity).

The important number here is a total amount of rolls by all affiliates, if most of those 8000 affiliates done just a few rolls, this isn't so strange.