Here we have a servo wheel with a pushrod connected. Pushrod is blue and connected at 12o'clock. You move your stick 15degrees and the servo moves the amount shown by the blue dots. Pushrod moves the light blue amount.

Now connect the pushrod to either of the holes at the red dots. Move your TX stick 15 degrees and the servo moves the same rotation it did just before. But the pushrod barely moves except sideways.

That's how differential happens at the servo.