Should heart-to-body energy consumption ratio equal oxygen consumption ratio?
At first glance there are a few major issues with your calculation and assumptions.


You're comparing the energy output in work (pumping action) with the energy consumption of the rest of the body in input (oxygen consumption). I don't know how efficient the heart is, but I'd guess only somewhere from 20-40% of input energy is actually converted to pumping action.  
Oxygen consumption is correlated with energy consumption, and would be a good way to measure energy consumption if you know exactly what the subject is burning. However, if you have a big difference between the preferred fuels between the heart and the rest of the body, this doesn't really apply  anymore. A quick google leads me here: https://www.ncbi.nlm.nih.gov/pubmed/17081788, showing that the heart prefers to burn fat over glucose, and this creates a significant difference: 


  Carbohydrate oxidation typically generates approximately 120 kcal per mole of respired oxygen, whereas fatty acid
  oxidation typically generates only approximately 100 kcal per mole of oxygen.



This means that the heart needs more oxygen than the rest of the body to generate the same amount of energy. 

I think combined this could explain the 5-fold difference.