Unfortunately, sexing chickens is not like sexing a human baby, a newborn kitten/puppy, etc - the genitalia of a chick is such that even with close examination there is a 5-15% chance of getting it wrong (and this is with trained professionals doing the examination) - which means that "sexed pullets" always have the chance of being male, with the exception of sex linked crosses and auto-sexing breeds. Unfortunately, you have won the cockerel lottery here - great time to be that "one in a million" person, huh?