The lack of air resistance

Studies showed that running on a flat treadmill burned about 4 percent less energy, but that difference could be eliminated by setting the treadmill incline to 1 percent.

Air resistance depends on how fast you’re going. If you’re running really fast, air resistance takes a bigger toll, which means that a really fast treadmill running is artificially easy by a larger amount. Conversely, when you’re running really slowly, air resistance is almost irrelevant.