How many calories does walking one mile really burn?

I have two main conflicting sources for how may calories I'm burning while walking, so I figured I'd turn to the public's answers. I'm: male, approximately 145 pounds, 5'8", and walk 3.5 to 4 mph (15 to 13 minute mile). Internet searches, my first source, which is probably aimed towards a general audience has shown me that I burn about 75 calories per mile however, my second source, a pedometer app for iPhone called Pacer, which allows me to add more specific values, gives me less than 50 calories per mile. Which one is correct, or more correct? Feel free to add your own knowledge.

Thanks in advance!

Added (1). @BDary2 Yeah. That was probably my cheif explanation, it just surprises me that it makes that much of a difference. Then again, where I live it feels like San Francisco with all the hills.

Its all just an estimate as it doesn't know when the terrain is slightly uphill etc. As that all matters alot.