[isoka]

There is a desynchronization between client and server mana ticks. It can be noticed by seeing over the course of 6 seconds your mana being updated twice. Any cast will actually refresh your real mana number (getting rid of the fake mana tick till the next one ...), especially instant cast clickies.

The fact that you end up with 221 mana after casting canni shows that you were standing when the real med tick happened (+1 standing) to which you received +20 mana from canni = 200 + 21 mana.

Also, if you want to make sure you catch the real med tick, pay attention to your HPs. It gets updated much more accurately than the mana pool. You will see your HPs acting weird with a little jump in numbers. But once it goes back to the expected number (starting HP + regen sitting), you can be sure the med tick just happened.