Minnesota right now has 50 points with six games remaining. If they win out, they will be at 68 points in 28 games; or 68 of 84 points won. That is, 0.8095
Prior to tonight's UW game, both Ohio State and Wisconsin were at 45 points with seven games left to play. If either wins out, they would be at 66 points in 27 games; or 66 of 81 points won. That is, 0.8148
Minnesota has the one additional game, but only got two of the three points available (68 points vs 66 points). So, only .666 for that game, dragging their overall percent down just a bit. (.8095 vs .8148)
Exactly the reason that tonight's win by Wisconsin has them in first place, 0.762 vs 0.758. One fewer game played but only two of the possible three points gained from the extra Minn game.
(Think of a baseball player's batting average: say he's hitting .350 for the season, and then plays a game where he only goes 1-for-3. He hit .333 for the game, lower than the .350. His season average is going to be pulled down a bit by the 1-for-3. Maybe .348 or some such. Same thing for Minn only going two-for-three in the extra game.)