Without knowing the threads that you're talking about, it might involve the amount of food required to grow. For instance, if you're trying to produce lots of settlers, you want to minimize the number of turns between growths, and that might often happen with less than 7 people in a city.
It could also have to do with keeping your city from rioting... in certain cases you might not want to have to make an entertainer or raise the luxury slider. But that would be very specific to your difficulty level, your city improvements, and your luxuries.