I can see where fan base is likely based on the franchise's success, but what does the "band wagon" represent?
Are these on a 0-20 scale?

bandwagon is basically how many people "got on your bandwagon" because you are dominating as an elite team or giving them a fairtytale underdog story. In clearer terms: this is how many "new fans" you are attracting. Some of those fans occasionally stay and get converted into 'real' fan base.

Numbers are 0-100 (but you can go a bit above even) - dont be disappointed if its low, that goes for almost all teams, and bandwagon will be almost reset at the beginning of every season.