The regeneration spell does not update the number of life shards you have after the spell has been set so if you gain life shards after you have cast the spell you have to cancel it and then recast the spell to gain the improved benefit. For example the courage spell gives the proper number of hp if you gain more life shards after you have cast the spell.

Noticed this when i discovered about  8 shards next to my pariden city. Funny, since i have very little expansion choices. was about to restart when i found a food resource nearby. Now i decided to run through the game as far as i can with one city fueld by my magical might lol die ai die.