Since you brought the rear up I'm not sure how you can install that wrong. Without customizing a bunch everything " falls" into place.
I will watch the video...
Yes, it does "fall into place", but you have to look at the whole picture.
We have to understand factory tolerances. That is how much the bits and pieces can deviate from "perfect" and still be good enough to put on a car.
The perches were welded on to the rear housing on a jig at the rear factory. There's a tolerance to where they are positioned. The front and rear spring mounts are drilled in the frame at the factory with a tolerance, then the frame structure is welded in place with a tolerance. Leaf springs are arched with a tolerance. On and on.
You can have what they call "tolerance stack up" where one piece is off the maximum, the next one is too, followed by the next one... and before you know it, the end result isn't quite right.
So, you could have the front mounting holes up a little too high, the perches off just a little, the spring arch not quite right... In theory, it's should be good, but let's say that the perches aren't right and were never caught by inspection. Now let's even make it more complicated and drive the car around for a few years, get new springs, have the car sag a little... Suddenly that angle might be bad enough to make a difference.
Hope that explains one scenario of why the angle could be off. That's not taking about crashes, changing rears, welding new perches, re-arched springs and on and on. I knew you were really wondering about how a stock setup could be wrong.