So somebody who knows more about physiology explain this to me. If Josee, Victoria and I are all running 10 miles at an 8mm why do they burn more calories? Because they are younger? I would think I would have a higher burn because I am heavier. What am I missing here?
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I am pretty sure that HR plays a part of this. I also think that even when wearing the HRM it is still an estimate.
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There are many reasons why one runner is more efficient than another - accuracy of reporting, age, level of conditioning, altitude, terrain, topography, body mass, weather, and nutrition just off the top of my head.

If you have good data (or even consistent data) you can see how calorie counts vary over the course of a single run. These are my Garmin 620 calorie expenditures* - 81, 111, 120, 118, 109. The pace for the 81 calorie mile was 10:30 while the other miles were 1015, 1013, 1014, and 1027. Overall, I burned 539 calories at an average pace of 10:20

Seven days ago I ran the exact same route with an average pace of 1024 but needed 604 calories (12% more) and my calorie estimates were 112, 127, 130, 122, and 113.

Same person, same route, same weather, etc., etc. and there's a 12% variance extrapolate that out to another person running and it's only expected that calorie counts could be very different or could be very similar.

" I also think that even when wearing the HRM it is still an estimate."
Yes, they're all estimates. The data from my 620 is, as best I can tell, ±10% accurate and, with $80 of software and a tweak to the 620, 610, or 910, you can get to about 5%±. Regardless, they're all estimates - the best indicator is when you step on the scale.

It would take me 12 miles to burn 1000 calories. It doesn't even matter how fast I go really or slow. I average around 85 calories a mile usually. I used to burn so much more per mile. I am 32 years old, 5'10" 137 lbs.

33 yo, 6'1", 200 lbs.

I did an easy 7 miles last Friday. 10 minute miles. Average HR of 139. 1,085 calories burned including a 10 minutes total of walking during warm up and cool down.

The Saturday before that I did 7 miles at about a 10:45 pace. Average HR of 131. 1,026 calories burned.

So I guess it's somewhere around 7 miles for me.

I'm 5'5" and weigh between 120 - 126 generally - and for me, I burn about 65 calories a mile, running between 7.00 pace and 8.30 pace. The pace doesn't seem to make any difference. It sucks.

The Half marathon I did recently, while wearing my 310xt with hrm, said 820 calories. Average HR 165, max 180. Pace 7.24. I can tell you I ate WAY over that amount for dinner the night before.....

Now I really am depressed!

I burn somewhere around 110 calories per mile according to my fitbit and my pace is usually around a 10-11 minute mile. I'm 168ish right now and 31

65 minutes, 7 miles, 9:15 avg pace = 907 cal. I am 53 yrs, 5'8 and 165 lbs.
Per garmin.

it seems to be around 10 miles for 1000 calories. 140 lbs female.