IIIF import: coordinates are unclear

The W3C json export for a processed IIIF manifest are neither absolute coordinates for the original IIIF canvas dimensionions nor for the internal 640px downscale nor for the 1000px exported JPG.

Example:

Export coordinates (not matching): https://www.e-rara.ch/i3f/v20/20950282/298,774,325,389/full/0/default.jpg

Approximation to correct coordinates: https://www.e-rara.ch/i3f/v20/20950282/490,1250,534,640/full/0/default.jpg