Marty has always been inexplicably protected from the changes in the timeline. Thus he operates with a memory of the original timeline in which the "time machine" was never invented (or never used).
In Back to the Future when Marty returns from the past; Marty is unaware of the following changes to the timeline that his meddling has caused:
- that his father wrote a book
- the socio-economic status of his family members
- the truck
These things don't make a lot of sense if Marty is traveling through time in the same universe, because the instant he changed something he should immediately be converted into the updated Marty which already knows all these things. But he isn't. Marty is just lucky he arrived in a universe where his doppelganger (the Marty native to that universe) also jumps in a "time machine" and disappears with a flash.
In Back to the Future 2 Marty travels to the future and his actions result in an old Biffy traveling back to 1955 and changing history. Subsequently When Doc Brown, Jennifer, Einstein, and Marty attempt to return to 1985 they think they are in an alter timeline. Things that suggest Marty is in parallel universes:
- Marty encounters himself in the future, but our Marty never returns from this future, so this future Marty shouldn't exist (he should have disappeared in 1985 never to be seen again, just like the original universe from which original Marty left).
- When Marty returns to 1985 there is another Marty in this world already. The one who is attending boarding school somewhere.
- In this universe Doc Brown never created the "time machine" and Marty never went back in time.
I'm thinking when old Biffy went back in time, he did so to another universe, one which he changed but for which events didn't depend on the creation of a "time machine."
I'm putting "time machine" in quotes because it now seems to me that the machine really travels between parallel universes AND time.
In Back to the Future 3 Marty travels to 1885. This same Marty (and Doc Brown) have knowledge of Clayton Ravine and its history, despite their meddling which causes this future to no longer exist.
Has Marty been hopping through parallel universes all this time, never returning to his original one?