It is because you are not trying to hide it. When you are alone you dont have to worry because there is no one there. Some people stutter in front of camera even when there are alone (not your case). That is because they worry someone may see it or notice something about themself they want to hide so bad. When you are at your job you dont wanna see as weird or awkward so you stutter. Is a defensive mechanism to prevent yourself from living your worst fear; being strange, unaccepted, different, unloved. It´s like people who have fear of failing and they voluntary sabotage they performance to fail or dont even try. If you dont try, you cant fail. **If you dont talk, you can´t be judge.** It is in deed a strange phenomena because stuttering itself make´s you look weird so it doesn´t solve the problem. But your mind creates the blocking just trying to protect you from "harm". The mind can´t measure the social effect.