......your servo horn is not centered. Remove the push rods and servo horn, place the transmitter (turned on) trim to the center, turn on the receiver, then replace the servo horn so that the arms are 90% to the push rods. Now replace the push rods and adjust to obtain neutral control surfaces. Measure the amount of throws and adjust push rods in, or out, on the control horns or servo arm until you get the throw you're looking for. This should get your throws very close to equal up/down movement.

If you want differential, as your book states, then it's an easy procedure. Just move the servo horn (with everything hooked up) one notch either CW or CCW and check the control surface throws again.

Hope this helps.