I have seen people fail @ 90% 4 times in a row and 15 times @ 20% and others get from mortle to Half Angel 4 with 3 fails. It all depends on the RNG then you unlock the hero. The same goes with stars got a 9 star khan with one fail and have a lance that is 7 stars and with 20 fails so far. As mattp said with the marbles, just because you have a 20% chance to pull a black one it does not mean a black one will be pulled 20 times out of 100.