JR 10X programmable mix problem!
Is there a reason that you are using a p-mix to do this rather than the built in function? The built in function in the Wing Type (shortcut 22) will do the same, eliminate the differential you are seeing, and free up that p-mix for other stuff. Go to Wing Type and select D/ELEV under the TAIL label. This will turn on dual elevators using the elevator channel for the right elevator, and Aux 3 for the left elevator.
Oh, and once you have the ATV's setup for the controls on separate channels such as dual aileron servos, dual elevator servos, and ganged rudder servos, use the Trace Rate function to adjust the travels, rather then the individual ATV's of each channel. The Trace Rate function will simultaneously adjust both channels in unison so you wont have to adjust each elevator half independently in their respective channels.
Since you are used to a 9Z, also note that there is a built in function for knife edge mixing (Rudder to Aileron/Elevator mixing - shortcut 64), so that you don't need to use a mixer. Its easier than trying to use the Rudder to Aileron function, and then a separate mixer for the rudder to elevator component as done on the 9Z.