I am also similar to this. For me, I am more calm in formal scenarios because there is an expectation of how to speak/talk/act. Everyone around you is on a level playing field and is bound by these expectations. Also, there is an assumed level of mutual respect in these conversations. In informal scenarios I stutter more because the flow of conversation is often widely different. People interrupt, make off the cuff jokes, and engage in banter; that is, conversation can be very unpredictable and require on-the-spot responses that I find difficult to blurt out. Moreover, people can be assholes with little consequence in these scenarios.