No, you got it right. Differential (more up aileron than down) is supposed to counter the drag of the down going aileron by moving the up going ditto more, enough to equal the drag of the down one. You will only need it if your model yaws with aileron movement. Fly the model away from you so you can see straight up the tail. Apply aileron (say right) If the model yaws left you will see the tail swing right, and you will need to add more up aileron than down. If the tail swings left, ie the model yaws right, then either you have too much differential, or you need to add more down movement than up to the ailerons. Most aerobatic types don't need it, most biplanes and high wing types need a lot.
Evan, WB#12.