color reduction reduces brightness of white backgrounds

[corr]

Trying to reduce a scanned document to 64 colors (dithering: none). However, the white background becomes darker. This is an effect that I cannot observe in Photoshop.

How can I preserve the white background in my image?

I can make a level afterwards, but it doesn't seem to me to be the best way to do this after color reduction.

[xnview]

could you post a sample file?