because your math is wrong thats how

assuming u pot a Super mana pot on each CD

if u get back 1800mana without a stone, u get 2520 with it a bonus of 720  "free" mana. Which over 2mins is 30mp5

if u get back 3000mana without a stone, u get 4200 with it a bonus of 1200 "free" mana. Which over 2mins is 50mp5

So at worst its 30mp5 chain potting, at best its 50mp5.

Surprise Surprise its a average of 40mp5 in the long run.

One of the BEST healing trinkets at the moment, and its not difficult to get.