Calories burned calculation

Art63

I've noticed that the number of calories burned for walking here on MFP is much lower than when I use other calory calculators. Why is that and which one is accurate? I'm not complaining about the lower number here and since I've been losing weight at a 1 pound per week clip since last summer I'm certainly a believer in the numbers MFP provide....just curious as to why the large discrepency in calculators.