The CORRECT use of the term "Channel" would be which servo you are trying to make move (Channel 1 = aileron, Channel 2 = Elevator.....)

The COMMON mususe of channel to refer to the Frequency numbers and thus the actual frequencies the radios use... I'm not sure how it got started, but it was a mistake. It started about the time of the new frequency allocations in the 1980's.

Before 1978, (maybe 1976.. close enough.) we needed a LICENCE to operate a R/C transmitter in the 72 mhz or 75 mhz bands. The massive amount of licence applications for CB's is one reason they dropped the requirement. The FCC expected the same boom in R/C transmitter use. (didn't happen.)
However the CB user's misnomer of calling thier frequincy numbers "Channels" may have something to do with how R/C enthusiasts began using the same misnomer.