I see what you're saying but a couple of differences. Mixon had a concussion in week 14-15, then left week 16 with an ankle sprain. But he returned week 17. He also missed 0 games I believe at Oklahoma.
According to sportsinjurypredictor.com, "Fournette dealt with an ankle injury throughout his final season at LSU, aggravating it 3 separate times and missing 6 games." Then of course he missed several games last year. Plus, in fantasy, he was drafted this year in 1st round while Mixon wasn't.