When an email reply is written specifically to someone besides you and nobody else needs to see this reply, the sender will hit “Reply All.” When a reply needs to be seen by everyone, the user will just hit “Reply.”
human nature?
Confirmation bias
Linda Ellerbee fell victim to this long before modern email. I believe she was working at CBS at the time, in the 1970s I think, and she sent a message out to everyone intended for only one colleague where she bashed management. Apparently it wasn’t taken too seriously since she continued on with a long career in news media.