Take a look at their conference only records. At the end of this season, Fleck will have coached one more B1G game in 9 years than Mason did. Assuming we beat both Northwestern and Wisconsin:
In 80 games, Mason coached against 32 ranked teams. in 81 Games, Fleck coached against 22 ranked teams.
If you apply Mason's winning percentage against unranked opponents to the same number of unranked opponents Fleck coached against, he'd have ended wih 38 B1G wins to Fleck's 40 (assuming we beat Northwestern and Wisconsin).
If you apply Fleck's winning percentage against Ranked teams to the same Number of ranked teams as Mason, his win total against ranked teams would increase by 3, but his overall win total would decrease by 3 since he won 6 against the non ranked oppoenents that replaced so Fleck would win that 37 - 32 (again assuming we beat Northwestern and Wisconsin).
So, yeah, Fleck has a slightly better record in the B1G over 80/81 games, when adjusting apples to apples. That doesn't account for playing games in the metrodome which hurt recruiting. The athletes village, and more dollars available to the team becasue of the TV contracts in place during Fleck's tenure.
I'm not saying Fleck sucks, but the numbers don't scream significant difference in actual on field results. While Fleck does have a higher winning percentage against ranked teams, his winning percentage against unranked teams is lower than Guteknust (-26%), Clayes (-11%), Kill (-6%), and Mason (-4%) while playing 4% - 13% fewer games against ranked opponents. That also can't be overlooked.
But if you can look at that and still claim Fleck is a far superior program leader. Go for it.