Thinking about the gramin/HRM thing, I seem to remember it goes balistic over a certain HR (or at least the calories burnt/minute is exponentail with heart rate)? So 2 hours plodding round on the road bike at 150bpm barely burns anything. 2 hours on the singlespeed and the calorie consumption goes through the roof even despite the average only being 5bpm higher (but probably far less constant).
So could it be that variation between people is down to how quickly your heart reacts to an effort i.e mine rockets up to pretty much it’s max, then drops equaly quickly once over the hill therefore the garmins registering more calories, whereas someone else might do the same ride but log a more constant effort?
I’d ignore all of them anyway, If I’m off for a long ride then I just aim for 400Cal/hour of cereal bars, jelly babies, dried fruit and energy drink. Then after the ride eat normaly.