Board Thread:The Quarters - Q & A/@comment-165.106.21.112-20160706182103/@comment-24672610-20160706203547

Those are the probabilities, but since each one is determined separately by random chance, there's no guarantee that the distribution you get will exactly match the probabilities (in fact, at relatively small sample sizes it would be surprising if it did match perfectly!). There's nothing in the code that would change those chances, so it's just luck.