College Football Defense vs Ranked Opponents

While on my favorite college football stats site, I saw you could segregate some stats based on ranked opponents vs non ranked. I am just getting started but this is what I see so far. The ranked teams score about 10-11 points per game above non ranked teams. This was constant since 2004. I am going to see how well defensive rankings (based on points allowed) change when comparing ranked vs unranked teams. Basically, how much do statistics against unranked teams matter when playing ranked teams?