I don't disagree with you on that, but this is really not that accurate.
I have fitted a pair of PW (only mechanism) from another car in my doors. My doors are just now without doorskin so they're very easy to work with compared to when they're fitted together with the doorskins again.
The mechanism is tested and works very well, but I need to make it stop on top where I want it. Since not the sidewindows or the front screen is in place I will make this stop point a little changeable. But the changeable range can't be too large. In order to put the chageable stop point area ROUGHLY in the right place I need to know this side opening.

Another matter is the fact that later this will be a question of design and things to fit or not to fit together. My bet is that this opening actually NOT vary very much from car to car! Especially not for cars with Duval wind screen.
Very little out of line and the whole car looks odd.

But of course. When the day comes to actually fit the top, or to fit the sidewindows to the front screen, then it's important to do things in the right sequence!