Erenous calorie calculation Endomondo/MFP

OK, I post my riding data to Endomondo so that it automatically syncs to MFP. For me this process seems to work just fine.
I see others that use the manual entry method and select the "Average speed range" for their ride and enter the riding duration for it. I went on a long ride today for 328 mins of riding and I get the following results:

Endomondo post to MFP "cycling 14-16 mph" for 328 mins Calories burnt 2,478

Entering direct into MFP "cycling 14-16 mph" for 328 mins Calories burnt 4,123

This is a huge variation! Can it not be calculated better than this?