Preparation doesn’t guarantee success. There are unexpected variables at play. For instance, your teleprompter may break, or you’re given an outdated document to reference.
But what preparation does do is set you up for improvisation. When you know your stuff like the back of your hand you can call an audible and still execute effectively.
Preparation creates flexibility. It alleviates the fear of thinking on your feet. When you’re confident and knowledgable, you dictate the result.