Ah, that’s a good point.
My only guess would be that things don’t happen instantaneously like they should. For example in the original Back to the Future, under a logical understanding of time travel, the moment the past was changed so much that Marty shouldn’t exist, Marty should have just disappeared. Boom.
Instead, Marty starts to slowly fade out. In the world of Back to the Future it looks like there is some weird stuff going on where major changes that happen at one point in time take “time” to propagate out.
Imagine if you would, that you could watch the “timeline” of Back to the Future from some sort of outer-dimension. A change in 1955 will create a new timeline arc, but it takes time before it “replaces” the original arc. I believe what happens is by giving the almanac to young Biff in 1955, Old Biff is actually totally destroying the original timeline that comes after that, and it will be replaced by a new one. (So the Doc’s drawing of a divergent timeline is sort of correct, except he fails to mention the original just ceases to exist entirely.)
Apparently those things aren’t instantaneous, and during the “propagation” time, you could still move back and forth on the original timeline, before it is destroyed.
Obviously that doesn’t make sense, but it does seem to comply with other instances of such things in the movies (like Marty being in a “race” to save himself in the first movie. Obviously if things were instantaneous like they “should” be, Marty would have just stopped existing the moment he got hit by his grandfather’s car instead of his dad.)