Connelly's pre-season sp+ numbers are based on returning production, recent recruiting, and recent history. It does not include polling.
You can see his full post here:
https://www.espn.com/college-footba...+-college-football-rankings-alabama-back-no-1
He beat the vegas spread at a 58% clip during the first 5 weeks of last season.
SP+ projects the spread on a neutral field. To turn them into a true spread, you just have to add 3 points for home games and remove 3 points for away games.
I assume the number that doesn't make sense to you is Nebraska. I would suggest taking a look at their 2-deep. Much like the Gophers, they return their entire offensive line and most of their offensive production, but they lose their defensive front and their best play-maker. They will be a good team next year, but they face a tough schedule.
They are, in almost every way, a mirror image of the Gophers with marginally worse coaching. We play them on Black Friday in a pretty symbolic game. They will certainly be up for it.