I believe the previous posters are correct. If you /who #channel1, the response will always show each nick as being on #channel1. However, if you /who Nick, the IRC server will return the first channel that the user joined on that server.
Even though you are typing /who Nick in a channel window, the IRC server has no idea which window you are typing in. The command sent to the server (/who nick) doesn't reflect the channel name in any way.
-genius_at_work
