Question about calories burned

jigsaw00

When I run on the tread mill or ride the exercise bike I'm given a total calorie burned number. This is always less than when I put the amount of time spent into MFP for the same exercise in the database. Usually its around 1/2 of what MFP says depending on what weight I put in for myself.

True weight and MFP is twice, at 100 lbs (over 1/2 my weight) MFP matches up with the machine...

Which number should I use, and better yet how does MFP pick a number?

I've been logging the machine number since it is lower than the MFP number as I don't want to eat to many more calories that I really should be...