Why the difference in duration?

[Vaer]

They adjusted the potency of the healing spells up for AST (they are quite similar right now if you look at equivalent spells) but didn't touch the mp cost. Probably same reason here. Made sense before because the spells were lower potency but the mp cost is still from the old balancing. They just never changed it so AST spells are cheaper to cast than WHM equivalents.

Who knows why. Probably to get people to play AST. Same with the Balance card buff.