The calorie "estimations" of cycle computers are nearly useless as they are more a feature added for marketing purposes ("it also counts calories"). They give numbers that high because it makes people feel good about themselves and the purchase.

If you want a much better estimation, get a heart rate monitor, that will bring you much closer to the reality (it's not perfect, but at least it's a decent approximation). To get an idea about the research behind that estimation, see
http://www.firstbeattechnologies.com...Estimation.pdf. Suunto uses this for its HRMs.