Hi j., as far as I know, it's not a deep dark secret, but I did forward this to moderators and other support. The quick answer is that the formula that is used here is similar to the ones widely used by others, but to get a definitive answer will take some digging. Given that the techs are pretty busy programming new and awesome stuff into the site, it may take a bit.
I agree that the numbers should not be 50% higher, but also keep in mind that the counters on the machines at the gym are widely thought to be rather inaccurate. Sometimes I've had the thought that it would be cool to just have a device we could plug ourselves into and it would automatically calculate calories eaten and burned! Using a heart rate monitor was as close as I got.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days