Calories Burned Question

I know generally online calculators based on activity are higher than the actual. I usually enter my cardio here, on runkeeper, and do look at the exercise equipment. I'm not too obsessed with the numbers, but these three are all pretty similar.
I have recently been keeping track of my heart rate via work out machines (I do not own a HRM). I was interested in calories burned based on my HR. I found these calorie numbers were pretty high. For example, MFP told me 300, Runkeeper with the elevation said 300, and without the elevation (as if I was stationary) told me 350. I plugged it into this:
http://easycalculation.com/health/heart-rate-calorie-burn.php and it says 500.

Any ideas? The example was cycling.