All calorie burns are just estimates. In the end, I never trust machines; they can be off by as much as 30%! The best way to get a semi-accurate estimation is while using a heart rate monitor with a chest strap, that tracks your weight, age, and gender. That's still just an estimate, but at least it's tracking actual exertion.
I wouldn't stress over pinpoint accuracy; outside of a lab you're not going to find it.