This is likely because some replies include the username as text inside of the tweet, you run across it with tweets that are old enough if any of the conversati

This is likely because some replies include the username as text inside of the tweet, you run across it with tweets that are old enough if any of the conversation participants did a nick change. This also means that if an account is deleted or renamed and then later someone claims the old name, it will look like the tweet was replying to them when it was really in reply to someone else. I think for modern tweets the reply-to is stored in a different way so that it doesn't count against the length limit and is only shown in the replying-to bits of the UI, but I'm not sure precisely how it works.