Mike Wardian would probably agree with this explanation. In 2004, Wardian set a treadmill marathon world record of 2:23:58. His outdoor marathon PR is 2:17:49, and he estimates that he was in 2:21 outdoor marathon shape on the day of his indoor record attempt. When I asked Wardian what limited his performance on the treadmill, he said that he felt "locked in" and that it was therefore more "mentally challenging" than running outdoors.
The bottom line is that if you want to make your treadmill runs as equivalent as possible to your outdoor runs, it's neither the heart rate nor the pace of your outdoor runs that you should try to duplicate indoors, but rather the perceived effort level. There's certainly no need to play with the belt incline. Just set it at zero percent and adjust the pace until your effort level feels the same as when you're doing the same type of workout outside. When you have your effort dialed in, you should find that both your speed and your heart rate are slightly lower than they would be if you were running at the same effort level overground.
In fact, this pattern has been demonstrated experimentally. In 1991, Swedish researchers asked 11 volunteers to run both indoors and outdoors at perceived effort ratings of 11 ("light exertion"), 13 ("somewhat hard"), and 15 ("hard") on a 6 to 20 scale. The subjects ran slower and exhibited lower heart rates at all three effort levels indoors. But because their perceived effort ratings were equal, they were getting equivalent workout in both environments.
Amaze your running buddies with this information the next time one of them tells you about the one percent rule!Sign up for your next race.