How to understand synchronism?
Not sure about how to find the numbers, but this has to do with the fact that not all loads are switched on at the same time. 

If you have 10 loads of each 100W, then the maximum power you can possibly draw is 10 × 100 = 1000W. But depending on the environment you are in, not all these loads are necessarily switched on at the same time. For example if I have a sander machine and an electric drill, I can't use them both at the same time. It all comes down to situation, experience and statistics.