(shrugs) I'm perfectly happy with the theory that time travel into the past always involves jumping to an alternate timeline, but in most cases the timeline is so similar to the one you left as to be essentially the same.
"Parallels" even establishes how you can determine whether you're in the timeline you started out in and one way you could return.
But if you always end up going back to an alternate timeline that is practically identical to the one you left, then wouldn't you already exist in that new timeline? You would run into yourself! That is unless "you" in that alternate time line also had gone into the past.
In Star Trek IV. When the crew of the Enterprise returns to the future they are obviously coming back to the same timeline they left. We see that because the destruction of the probe is still going on.