I've noticed this as well. I raided/dungeon/pvp'd yesterday and made about 15 of these because I had extra deepstone oil. If you have an alchemist stone equipped, these are by far the best mana regen items you can get. I had times when they would heal for 20k(mana), and times where it would be nearly 40k mana.
20k mana is higher than any other potion except for concentration - which has limited uses in raid environments. I reccomend using these for all casters if you have an alchemist stone on.