Vitality 1 and 2 stack but Vitality 2 and 2 don't

I can put Vitality 1 and Vitality 2 on two different spirit gems and they work i.e. their regeneration is applied in character stats and actual gameplay. Vitality 2 and Vitality 2 can however not be put into two different gems. I assume that the latter is the intended way of working for this so the question arises if the 1 and 2 stacking should even work...?