I was taught that the parshiyot are named for the first significant word (or, rarely, phrase) in the parsha, and obviously names have to be unique. This week's parsha begins "eileh toldot Noach" and is named "Noach". Later we'll get one that begins "eileh toldot Yitzchak" and is named... "Toldot", not "Yitzchak". (Edit: I said "Yaakov" originally, oops!)
Why is this? Does the later one, being about a patriarch, claim the earlier word "toldot"? Is it because the entire Noach story is contained in the earlier parsha so we name it after him? Is there some other reason?