There was a man called jesus who fed many on a fish finger and pitta bread !! so many factors to consider for yields. the stated ones are for optimal conditions and perfectly fed and grown. We never get that.lol Training is the key to yield initially and good lighting , feeds, environment ect... It takes time to hit it right and lot of research on other grows . dont be disheartened , be positive that the next one will be better and more productie too.