From Brian Samson:
""Ok, here's the part that confuses me - when you adjust your turret, it changes the angle of the internal lenses, but it doesn't offset them. So if you look through the centre of the ocular lens, the light path through all of the lenses in the scope is still through the centre of all the other lenses through to the objective, just at a slight angle.""
My take on this is the turrets manipulate the erector tube that carries the reticle and a couple of lenses, the erector tube is not angled but manipulated in the vertical and horizontal plane by the turrets.
When you optically centre you place this erector tube and its associated lenses in the centre of all the other scope lenses.
Willing to be proved wrong tho