"Because Azurill's gender ratio does not match that of its evolved forms and its evolution is not dependent on gender, being female 75% of the time while Marill and Azumarill are female only 50% of the time, female Azurill had a one in three chance of evolving into a male Marill prior to Generation VI. Whether or not this will happen is determined by its personality value. Azurill is the only Pokémon to do this." -
http://bulbapedia.bulbagarden.net/wiki/Azurill_(Pokémon)