When stroking paths the attached example illustrates that fitz obivously is darkening the entire enclosing bounding-box pixels by at least one when rendering paths. It is unclear if this is when compositing it with the background or earlier though.
Created attachment 5356 [details] lines.pdf Minimal example that strokes diagonal lines once or repeatedly.
Worth noting is that evince 0.8.1 (poppler 0.5.4 really), renders this as having multiple strokes grow fatter and wider, xpdf 3.01 on the other hand renders all lines identically. Of course mupdf is way off since it is affecting the enclosing bounding-box.
*** This bug has been marked as a duplicate of 690631 ***