Because I didn't think Northern Illinois was that good. Purdue was a two TD favorite and looked very good last week in Eugene, OR.
Just my opinion, but I think that's a big upset. You obviously don't think it was.
EDIT: Anytime a MAC team beats a decent Big 10 team, I think it is an upset. Just like the Central Michigan/Michigan State game last week. Purdue and Michigan State were both heavy favorites for both games.