Farm production

I'm playing it right now!I have 13 wheat farms with 10 workers each and ONE granary, BUT the granary never gets even half full !!! My city has only 700 people and 13(!!!) fully staffed wheat farms that produce only enough to half city. Do the math, man, it is NOT "fixed".

I know the game says that farms don't have workers when they actually do, but some farms with full staff DO work like they have only 1 employee. And cart-pushers are slow, even in 100% speed.