The only way to PROVE how good you are is by your competition.
Teams Alabama has played that plays into their YPP. Texas A&M, Georgia, Tennessee, Auburn. All top 25 teams. The other half of their schedule is not rated but they have significantly more talent coming from the south. What makes you think Northwestern would slow down Alabama better than the horses those teams are running out on the field? Pipe Dream.
Northwestern does not have the horses any of these teams have. Northwestern is slow and not athletic enough to compete with those teams. Alabama would score 50 on Northwestern. So would Ohio State and Clemson.