I believe to be classed as a viewer you need to be running the video.
To be classed as a chatter you need to be signed in.
These 2 things can be different.
I can watch video when not signed in. I can also stop video playing and stay in chat. Some people pause video when in game for example and they remain in chat but do not get counted as a viewer.
Also when you host you send viewers but not chatters.
To chat they must physically visit the persons stream, otherwise they stay in your chat.