It's a little quirk of the OI from days when the team numbers weren't so high. Whenever the highest bit is set, then the channel control dips are active.
You can use whatever channel you like. When it is first turned on the robot controller will search all channels for an OI broadcast with your team number, then lock onto that channel until it is powered down.
For directions on setting the channel see page 16 of the OI Guide:
http://www.ifirobotics.com/docs/oi-ref-guide-5-8-07.pdf