Message length in "reply to" status update



I am just starting with the REST API and a and am a bit confused. According to the documentation in order for the reply to work @username of the user I am replying to must be included into status update text. But when it is included - it counts toward toward message length, i.e. @username + message should be <= 140. But this is not the case when I am replying manually via twitter official app or website. In this case @username doesn’t count and only message itself should be <=140. Am I missing something? Thanks!