Natures Bounty appears to be adding +1 food per city level to a city based on when the spell is cost (not updating as the city grows).  For instance, if you have a level 2 city and you cast Nature's Bounty, it gives +2 food and does not update as the city grows.  So later the city is level 4 and it is still only getting +2 food.  If you then cast the spell again (it stacks) on the city it would then get a +4 for a total of 6.  If this was intended, just let me know.  Thanks