Clemson won last year, the 2016 season, but it was the Jan 2017 championship game. Gave Trump #1 jersey.
Alabama won season before that. 2015 regular season but Jan 2016 championship game. Gave Obama #16 jersey because they were 2016 champions.
Technically, Alabama should've given Trump a #18 jersey this year for their 2018 championship, but they didn't because Trump probably asked for a #17 jersey. It's a subtle wink and nod to us folks. Don't over complicate it