Doc and Marty went forward in time from their 1985 to 2015 to make sure Marty Jr. didn't go to jail. Biff went back from 2015 to 1955 to give himself the sports almanac, creating a 1985 where Doc never created the time machine, thus negating his ability to give himself the sports almanac.
This bothers me. I feel like somehow finding this paradoxical cheapens the whole Back to the Future franchise for me.
And I really like it when I splurge on non-diet microwave popcorn.