How to calculate lithium-ion battery energy density
My suspicion is that the published specs have been greatly over estimated. Also, I believe that the watt hour calculation that you have done is slightly overestimated as well.

The 12 volt lithium ion battery has a nominal voltage of 11.1 (or possibly 10.8 or even 11.4 depending on the specific lithium technology used). I base this on a nominal voltage of 3.7 volts for each of the cells, 3 cells in series to make the 12 volt battery.

So technically it is 11.1 * 6.8 = 75.48 Watt Hours. This equalls approx 466 Wh/KG. 

Not a big difference but a little less. 

I suspect that the battery is significantly less than 6800 mAh or possibly it weighs more.