Two things here.

I just got back from an indoor run, half of which was on the treadmill and half of which was on an indoor track. At the same perceived exertion easy rest and heart rate (145-150) I run about 20 seconds faster on the indoor track, 6:40 vs. 7:00. I've always thought the treadmill was harder.

Second, not all treadmills are the same. I switch at the gym sometimes just for the heck of it and I would guess that they are precise within .6mph from my experience. Thats a big difference to me.