I don't imagine that he is locked in at 8min pace the whole time. I bet there is a fair amount of 10min miles progressing to faster at the end.
I'm a 2:35 marathoner and I often run 9:00min pace recovery runs. I also only recently started wearing a Garmin. I used to look at my watch at the end of a run and I would note that I averaged 7:47 pace or whatever. Now that I wear a Garmin, I realize that I run 9:00 to begin with and 6:30 at the end but the effort is similar. Not saying this is what Mutai does, but I imagine it's similar.
Final thought, it's much easier to run slow if you run 145miles in 6 days.
I noticed the same thing since wearing a garmin. I may average 7:40 but at the end I'm going much faster with the same effort.
I figure he's running about 140-150 per week. A lot of "easy" miles- that's a good thing.