It's true that running on a treadmill is easier, mainly due to the lack of wind resistance (which is a bigger factor for running than you would think), but also due to the artificially perfectly flat surface; you have to make far fewer micro-adjustments to the terrain when you're on a treadmill, and like everything in distance running, those adjustments add up over time.
It's said that putting the treadmill at a 1-2 degree incline can correct for this discrepancy, and there's actually been a few scientific studies on this as well (e.g. http://www.ncbi.nlm.nih.gov/pubmed/8887211).
