It is a combination of 1) winning 14 road games 2) only losing 1 home game 3) playing only 12 home games and 4) playing a schedule that has a number of teams with good winning percentages (regardless of the teams' respective RPI rank). The RPI ranks of their opponents is less important than the above.
I said earlier, Indiana State is the post-child for overachieving w/respect to RPI. They are not nearly as good as their RPI says. This was true last year as well.
Brian