A calorie is equal to just over 4 Joules (actually 4.184), so when you take into account the body's efficiency of about 25% the number of Joules of energy output is a close match to the number of calories input. But, as stated before, the 25% efficiency is only approximate and does vary with cadence. Studies I've seen indicate that efficiency is highest at a pretty low cadence (around 60 rpm), but that results in faster muscle fatigue so most cyclists do better at a higher cadence (around 90) even though it's a little less efficient. Your lower heart rate is confirmation that you are a bit more efficient at the lower cadence and therefore your body's calorie consumption would also be a little lower.