Forerunner 35 shows different mileage, calories and speed to Reebok treadmill. Why?

Pace / distance is different because different methods are used for measuring the distance, not to mention accuracy issues.

Calories are different because your watch knows more about your physiology than the treadmill.

Thank you for your reply. Funnily enough, today, the calories were almost the same on watch and treadmill. Forerunner 35 gives about 10% higher speed and distance. As you say, they are both estimates so there may be inaccuracies on both sides but one would have thought the treadmill should be more accurate on distance since it determines the belt speed. I prefer the Gamin readings, though!