OTP differences are primarily a function of schedule padding vs. location of delays.
Schedule padding is approximately as follows.
#7: 45 min into MSP, 30 min into Minot, 30 min into Shelby, 15 min into Whitefish, 45 min into Spokane, 30 min into SEA/PDX
#8: 30 min into Spokane, 20 min into Whitefish, 30 min into Shelby, 30 min into Minot, 45 min into MSP, 30 min into Chicago.
Those numbers may not be entirely accurate, but they add up to 3:15 padding westbound and 3:05 padding eastbound - approximately equal.
The main delay agent these days is slow track between Fargo and Minot (especially the stretch from Fargo to Grand Forks), which results in approximately 90 minutes of delay each direction.
If #7 is 90 minutes late into Minot, it still has two hours and 30 minutes of schedule padding ahead of it, which is usually enough to make up the delay entirely and even arrive early into SEA/PDX.
If #8 is 90 minutes late into Fargo, it has only one hour and 15 minutes of schedule padding ahead of it, and thus is often still behind schedule when it gets to Chicago.
Mark