Interesting case you have here. Clearly there is something here. Are the grey areas in the right image from scratches in the ruler? Or from dirt on the ruler?
Could this be a saturation effect? In the sense that, if you push exposure on the right, the dirt will be less visible?
In microscopy we use a technique called “oblique lighting” to increase contrast on geometric features, like roughness. Perhaps your AL-MX has more oblique light?