JR & Futaba transmitters will have this problem when you mix channels. I am not sure exactly why this is so, but is has something to do with how the packet of information is put together and transmitted to the receiver.

What I have done is adjusted the end points and linkage to get the controls to track evenly though the entire range of motion when the controls are moved slowly. This seems to help. Also using high speed servos help as well (digitals are great).

To fully eliminate this problem a matchbox or reversible "Y" can be used. I don't like using these but I know of several people who do.