There is a lot of confusion over what a team commits to when they start a Sprint, and particularly the role of the Sprint Goal. I've previously covered that in longer posts, but the metaphor below may help to 'connect the dots' in a simpler way.
Think of it like hiking through the countryside. You choose a place where you want to camp at night on your map. Getting there is your goal, and you must get there before nightfall (i.e. a timebox).
You don't want to necessarily have to plough ahead at full speed without breaks to get there before nightfall, so you build in a little bit of slack time. You know from experience how often you need rest stops, or want to stop to look at scenery, etc. and you know roughly how quickly you can travel over the type of terrain that you expect to face.
You probably have a route in mind having looked at the map, but you know that there's a chance of some ground being marshy and slow to walk through, and other parts being impassible. So, as you progress, your plan is subject to change. You frequently inspect the progress that you've made, and the path ahead and make alterations to your plan based on what you learn. You may choose to lengthen the route to pass by some particular scenery if you are ahead of your plan, or shorten it if you are behind.
You continue to monitor progress towards the camp. If you realise that you might not make it before it gets dark, you need to consider changing to take you to another campsite, or maybe even returning back home if your reason for delay is injury, or stormy weather. If you do decide to head for another campsite, you may choose one that is in a completely different direction from where you were initially headed, based on what you've learnt and how things have changed.
The lesson here is that the goal is what you are committing to (the camp site) but the Sprint Backlog (means of getting there) is subject to change as a result of frequent inspection & adaption.