It is simple math. The copper lines that power is transmitted through is a good conductor of electricity. Even though it is a good conductor of electricity there is resistance in the copper. So there is a general rule of Loss per ft or meter depending on where you live. You also have allote of loss with Degrading the power to convert from HIGH Voltage 40,000 v down to 10,000,3000,1000,480,220, 120 three phase, single phase. Every time you step down you have a loss because the rule of the universe is that for every reaction you have an equal and oposite reaction. And this means energy and it means a loss. And this all before power even reaches your home or business. Now that you have the voltage step down it is less efficent in it;s transmission so you have loss there aswell. And lets not forget municipal grids are usually at 85% and you have some more loss there aswell. But it's all acceptably and all these calculations are figured in there business model.....

Anyway i could keep on going but i dont want to sound like a nerd