ToME: the Tales of Maj'Eyal

I have a maxed heavy armor training, and it seems like wearing a massive armor gives me 9% more armor hardness and 6% more crit reduction compared to when I'm wearing a heavy mail. But I don't see this bonus listed anywhere. Is it some kind of hidden bonus from the armor type? Am I missing something?

Yes, it's a hidden bonus. Massive armours have an extra multiplier for those bonuses.