However, chat channels span all of the channels...
Strictly speaking, not true.
"Zone" and "Say" chat are specific to whichever channel you play on, be it 1, 2 or 3. "World", "Party" & "Guild" along with any custom chats will span the server channels.
Interesting stuff happens inside instances with the "Zone" chat. The "Zone" chat will span all of that instance meaning that several parties all within their own copy of that instance can talk to each other in "Zone" chat and share any tactics or comments. This behaviour also occurs inside your House - you can use the "Zone" chat to talk to others who are inside their own houses.