Great, so let's use that as an example. As it happens, I'm currently replaying Fallout 2. I've played through it many times since it came out and while I can't claim perfect knowledge, it's quite vast (and probably a lot more detailed than what most GMs could come up with if asked to explain everything that's going on, I would guess). By your logic, that should make the game basically a railroad, but it very much ain't. Because I play a different character, because I like trying different things, because I'm a different person and probably a few other reasons, but the short version is that just because I know the game, the setting and the plot very well doesn't make it any more railroady than the first time I played it, since I still have as many (or as few, depending on how you view it) options as if I didn't know the first thing about it.