All of that is based on statistics. The only way to really know how many calories you are burning is in a lab, with gas exchange. It measures how much oxygen goes in and how how much goes out, as well as how much co2 is produced. From that you can determine how many calories you are burning, as well as how much of that is fats vs. carbohydrates.
Edit: Even a power meter won't tell you how much you are really burning, only how much work you have done. To convert from mechanical work (kJ) to food energy (kCal) you have to make an assumption about your efficiency (typically 20-25%). Note that if you do the lab test with a power meter you can determine your actual efficiency...
A physicist I ride with says that what with unit conversions and efficiency factors, kJ about equals kCal. He says it's plenty close enough. Take a look at your records and see if that seems right to you.