What do the "Energy Impact" numbers displayed in the new OSX 10.9 mean and how are they calculated?
I don't think you would want this numbers to be percentile and adding to 100 at any point because that would mean that your battery would die nearly instantaneously. 

If I was to make an educated guess just to help you out a bit, your computer (specifically MacBooks) have their battery capacity measure in mAh or milli Ampere-hour typically somewhere in the ballpark of 6500 mAh or 6.5 Ampere-hour. I would hazard a guess (if the moderators allow it) that your are seeing the actual usage to the second of how many mA are going to the specific app per refresh (call time by the Activity Monitor daemon).

So if you were to add up the average power draw of your active apps and multiple that per second you would know how long your computer battery would last without power (roughly). Additionally you have to factor in what internal components are working hard (AirPort, Bluetooth, Display brightness, hard drive rotations) and you have yourself the estimated time until depletion.

Oh, and as to how they are calculated. There is a tiny integrated circuit on your main logic board called the SMC and it's job is to allocate DC current to the resource that needs it most. It keeps  track of energy usage and demands by the RAM, CPU, GPU, etc. so that it can help make sure your computer stays energy efficient.