The difference is described in the ANSI standard for Voltage. I don't have it in front of me so I can't tell you the standard number.
But in a nutshell, they have two voltage levels. Service Voltage and Utilization Voltage. This recognizes the fact that you will have some voltage drop in...
You will also find that buying a single piece of anything, even after discounts, is a lot more expensive than buying that same thing as a part of an assembly or package of equipment.