Why do I not stutter at all when I'm alone?

Why do I not stutter at all when I'm alone? Hey fellas, I have a question. I usually stutter an average amount of time when I talk (not super badly, but noticable), however I recently noticed that when I talk to myself out loud or read to myself when no one's around to hear me, I don't stutter AT ALL. I'm freaking out when I have to speak up in class because I know I'll stutter constantly, but I just read an entire page full of words starting with sounds that I usually have difficulties pronouncing without even a slight stutter. Does anyone else have this weird form of stutter?