Accuracy of calories burned while biking

[nscrbug]

So I've been using my Garmin Edge 500 for a few weeks now and really like it. I think it's really cool to upload my rides to Garmin Connect and see all the ride data and graphs. The one thing that has me a bit skeptical is the amount of calories burned. I'm just not sure that I trust it, and I'd like to get opinions from others that are using the 500 to see if their results are similar.

I wear the HR strap (and yes, I do have it on correctly, as I've read that this can make a difference), and I wet the sensor patches on it before putting it on. I've entered my profile information correctly - weight/height/gender, etc. So here are the stats from my last 2 rides -

Ride 1
Time - 04:33:17
Distance - 71.05mi
Elev Gain - 659ft
Ave Temp - 56.5F
Avg Speed - 15.6mph
Avg HR - 108bpm
Max HR - 132bpm
Calories - 771

Ride 2
Time - 04:41:32
Distance - 73.33mi
Elev Gain - 1,745ft
Avg Temp - 76.5F
Avg Speed - 15.6mph
Avg HR - 109bpm
Max HR - 144bpm
Calories - 862

FWIW...I'm roughly 157lbs + bike weight (5'6" & 45 yo). I don't know, but the calorie burn seems awfully low for 4+ hour rides. My average HR is low too, which seems odd especially since I feel like I'm working SO hard (it was windy on both of these rides). I can't seem to ever get my HR very high when riding, as is clearly displayed by my MAX HR on either ride. Yet the effort I put out feels like I'm not just pedaling at a leisurely pace, I'm fighting to keep up with my riding buddy who is quite a bit faster than me...so I don't get it. It's kind of discouraging in a way...to feel exhausted/sore after a long ride, knowing you worked hard...then to see the results of your efforts, only to discover that the numbers are disappointing. I can burn an equal amount of calories at the gym doing cardio and weights in half the time it took me to burn the same amount on a 4 1/2 hour ride. I'm confused by this.

Linda