Just rereading the book.
It occurs to me that priests are a kind of celestial piramid scheme. The lower echelon believers pass mana on to the lower priests who take some, but pass the majority up, through their links, to higher priests, who gather it and pass it still to higher priests, who eventually pass it along to their god.
I can easily see a mid level beurocrat god saying “you cast 10 cure light wounds spell this months, that is 3 over your budget of 7 spells, but over all I saw your gross mana production drop by 3 percent. Will the added cure light wounds that you used increase your overall prayer to follower ratio, or will I have to consider letting you go and replacing you with a graven image?