You can have the whole thing prepped down to the floorboards and still have no idea what's going to happen once the players get involved.
I mean, take a typical old-time adventure module. With a few exceptions (most of which have numbers starting with DL) all they present is a setting and a bunch of maybe-connected situations and-or obstacles and-or potential opponents. Even if I-as-DM know this material inside out, I still have no way of knowing how (or even if!) the players/PCs will approach it or what will happen if-when they do.
Or take another example: the common situation where the DM has a behind-the-scenes schedule of events that will happen at specific times unless the PCs somehow force that schedule or those events to change. For example, I could have predetermined that two armies are going to meet and do battle in the Vale of Flowers on Day 12, but I've no idea how or if that's going to affect the PCs nor do I know how or if the PCs will affect it. Maybe the PCs broker a peace on Day 10. Maybe they're in the Vale for other reasons on Day 12 and unexpectedly get caught in the fighting. Maybe they'll work with one of the armies, helping them plan for that battle. Maybe they don't get to the Vale until Day 14, when all they see is flocks of crows. Or maybe they go another way entirely and don't hear about the battle for months.
I'm kind of in that situation right now. The party I'm running is in a hilly area that's on its way to becoming a war zone. Going in, I thought I wouldn't have to worry about details of the war itself as it was months away and the PCs should be long finished in the area by then. Well, what I initially thought would take them several in-game days (or at most a few weeks) has now gone on for three in-game months, meaning that armies, scouts, and strike forces are now moving in the hills around them like chess pieces.
I'd initially thought that war would be an off-screen bit of news. Now it's looking like I'll have to run it in some detail, if only to determine how or if it directly affects the PCs and-or what opportunities they might have to get involved if they so desire. And though I've got ideas as to who will win the war and how this will happen if all else remains the same, I've no idea how it's going to play out as regards the PCs nor do I know whether the PCs will intervene somehow and make a difference.