I have a weaver/pic rail set up in as mock on a 20 inch block I then place that in the vice set that level x,y then set the scope up on that . I know the base is right so it will be the same on the rifle also there is't cant to fix .
Nope, Weaver and Picattiny bases aren't necessarily precisely straight with the bore. And the rails aren't necessarily exactly straight, even before they are screwed down onto a receiver. The integral ones that are machined into the receiver have engineering integrity. As do the proprietary mounting systems, designed to work together precisely. But that still requires the barrel to point in precisely the same direction as the bullet comes out of the barrel.
Bases glued and screws Loctited will be consistent but not necessarily straight or even slightly twisted. But yes, decent mounts onto such a rail ought to be central left to right and repeatable on and off. Unlike things relying on dovetails, unless using the manufacturers' own good stuff, Optilocks etc.
Cant is important to understand, irrespective of the mounting system. As soon as there is some significant drop, a little bit of cant, i.e twisting the rifle so it is not completely vertical, nothing to do with offset of the mounts, will move things sideways, quite a lot. Even for "flat shooting" calibres. Particularly if you are using large objective 'scopes mounted high above the boreline. Yes they extend the distance between primary and secondary zeroes, but any cant can mess things up sideways, if you are not in control.
You don't need a centrefire to experience this, once you take say a rimfire to 100 m, or an air rifle to 40-50 m, it becomes quite significant.
If wanting to really evaluate 'scopes, you could set up a sort of optical bench with the test 'scope looking into the objective of the reference 'scope. It doesn't need to be anything fancy, nor does the reference 'scope need to be anything special, just an objective parallax adjustable airgun one, two sets of cheap mounts, and a bit of plywood or metal to screw them together. Test 'scope looking into reference 'scope.
Lets you see how well the adjustments work, backlash, repeatability, zero shift as you zoom, and real turret adjustment range, eye position parallax error and other such things.
None of them are perfect. But some are tat. Even a few of the expensive makes who should know better, or just made a bad one that slipped through QC.