"I still don't grasp the fact that the front mount does not move"
You want to change the angle of the scope with the bore. This is easily accomplished by moving one mount. If you needed to correct 4" at target for instance if you moved both mounts, then you would have to shift the scope by 4". By moving only one mount then a very small movement will give you the correction necessary.
Miller

Miller/TN
I Didn't Say Everything I Said, Yogi Berra