Played a few games this evening and it would seem a bug is allowing some players to recieve a fifth skill. In the games, players picking Freezing Field and Epicentre/Burrowstrike appear to get Frostbite and Caustic Finale as extra skills that can be leveled up.
Obviously this has game breaking potetial, as heros can skip 'useless' talents specific to other abilities to get a whole new skill.