I use Strava on my iPhone to log my daily bike commute to and from work. I then use tapiriik.com to sync my Strava account to all the various sites I use (Runkeeper, Endomondo, Garmin Connect, etc...). I noticed today that there is a huge difference in estimated calories burned between Strava and the other sites. For example, this morning I rode for just under 50 minutes and went 11.2 miles. Strava calculates that I burned 281 calories during the ride, Runkeeper estimates 526 calories, and Endomondo estimates 555 calories. All three sites have the same info in terms of my height/weight/age and they are all reporting about the same distance/time/average speed. I know that all of these sites can't give anything more than a rough estimate without HR data, but why would Strava be estimating so much lower than the other two, and which is most likely to be correct?