The scanner slows down and makes a longer scan to get more brightness into the capture.
With the camera re-shooting you change the exposure to get the brightness.
I would start with the scanner and see if you can make an adjustment. you might just be very suprised.
I am surprised already now.
I have never noticed that my scanner, ( it does not matter, witch scanner) slows down to get more brightness.
Yeh, sometimes he slows down. But the reason of that I know.
That is because I have set my HP Scanjet 5100c to scan on 600 dpi. ( optical)
He scan's each part ( a quarter inch , I think) 3 times. But mostly I scan on 300 dpi, what he does directly.
So you told me that my Primefilm 1800u scanner will slows down.
Never noticed that. He scans on 1800 dpi, optical. The end result is comparible with a 4 MEG pixel-camera.
So you understand now , I am surprised and curiously, the same time.
I will do to day a test with the primefilm 1800u scanner.