Why do people do this?

yknow what pisses me off? It's when just about anything I say will convince people that i am angry, and i have to watch every single precise word i say and I don't have to watch my anger as a result because these people are sooo gullible that i could say something that sounds calm while feeling pissed off and get them to think I'm calm as a result and that goes the same thing with the other way around if i say something disagreeing, or something that's very long to type, or just about anything, while feeling calm i could definitely get these people to think that i am angry without feeling angry but normally when i tell them this they don't even understand a word i said, so they just assume that I'm angry strictly and only because of what i said, which proves my points, and they assume that i am claiming that i am not angry which isn't true and i never said, but hey at least they are proving my point