Everything you said is true, until you look at each of the three games you referenced.
Purdue game weather limited Purdue's offense, Maryland was missing their top two receivers to injury, and Martinez is Nebraska's only threat.
Now, NW has injuries to key offensive guys after losing their best rb before the season started, Illinois loses their starting QB for the season, and Iowa wins with defense.
The "stars are aligned" for the Gopher defense to put up some really good stats in the next three games.
Sometimes a unit just gets lucky.
Good for us fans.