I don't follow that reasoning myself. Do you have a heartrate monitor? Could be that if not, but the treadmill has one... Or maybe that since the treadmill forces you to maintain a particular speed, that you'd keep it up longer than on the track, where you might slack.

Or, it occurs to me, using walking for cardio can be tough because, well, once you've been doing it for a bit, it's just not that challenging. But on the treadmill, you can adjust the incline, just making a walk far more challenging than it would be on a flat track. Could that be it?

(Out of curiosity, why no running?)