Calories burned calculations

I just noticed that my 48 minute bike ride this morning (15 km) burned fewer calories (388 calories) then a 44 minute "run" at lunch today (4.5 km and 409 calories). When I convert the run to a walk the calories burned is less (221 calories). None of these value match up with what my Garmin gives as calories burned values which is not all that surprising.

What does strike me as odd is the difference between the run and walk and the fact that the run results in more calories being burned then the bike.

Is there something wrong with the calorie calculation? Is there a way to see what equations are used for each activity?

As an added comment - it would be nice if you could account for the weight of your gear i.e. the 65lbs of Chariot carrier and child that I regularly push or pull while exercising. Is this possible and I just can't find it?

Mike