Many others have ventured their opinions on what the physics(or perceived physics) of the matter is. So I’ll offer a subjective opinion instead. From my perspective, treadmill running differs because your weight/most of your mass stays stationary, while you run your legs to maintain position. This works the muscles differently to planting a foot in front of you and propelling your weight forward through that rotation. Because on a treadmill the ground moves away instead of you moving forward, that rotational effort or propelling effort has to move half the distance it would in real life. The treadmill does part of the work for you by drawing your foot backwards.

(Hope that made sense)