It looks like that metric really likes the Majors and Bruce era from the 1970s.
Because the Big 8 in the 1970s was tough.
1971 - #1, #2, #3 in the final AP poll (ISU's 3 regular-season losses)
1972 - #2 and #4 in the final AP poll; 6 league teams ranked at some point (Top 20) during the season
1973 - 4 teams in the final Top 20; ISU and KSU only teams not ranked during the season; 6 of ISU's 7 league games were against a team ranked on game day
1976 - 5 teams in the final Top 20; 7 of 8 ranked during the season
1977 - Only 5 teams ranked during the season, but OU/NU/CU all cracked the Top 10 at some point