Possibly because you tend to stick with the treadmill longer - exercise more moderately but for longer periods at a time.
The treadmill is more work - but also more impact on your joints.I usually do an hour of cardio - split up between the elliptical, treadmill, and exercise bike.(Then I go circuit train with weights/machines).
I think the treadmill because I m always very sweaty afterwards. They both give you a workout so I use both.
