The description of the variable may give the explanation to this riddle, especially this part:
The variable returns values with opposite signs in the Floor Plan and the 3D Window. If you look at Chapter 12.1 in the GDL manual you will see that the way a Window/Door is generated in 3D is that it is rotated 90 degrees around its own X-axis. This way a direction that was positive Y on the Floor Plan Window becomes negative Z in the 3D Window and a direction that was negative Y on the Floor Plan Window becomes positive Z in the 3D Window. So if the variable returned 1.6 meters in the Floor Plan Window it will return –1.6 meters in the 3D Window.
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Thanks Laszlo, it makes sense from a strict geometry perspective. But it's crazy-making for a user. It is consistent, once I reversed my interior/exterior logic in the 2D script. I don't know how I missed that page while searching, but I very grateful for you pointing it out.