There is one thing in the Back to the Future movies that breaks all the rules set by the series. Biff steals the time machine in 2 to give the sports almanac to his younger self and returns it to the same timeline for Marty and Doc to use. Biff should have returned to a timeline where he has become rich, and Marty and Doc are trapped in 2015, at least until Doc can build another time machine.

I think what this really means is, all time revolves around one Marty. Which, of course, is totally weird. But it does explain why there isn't a second Marty wandering around somewhere at the end of the first movie. And Doc really isn't pretending to nothing from meeting Marty in 1955.