The resolution of saved PNMs is hardwired to 72dpi. Is there a good reason for that? If not, I would be happy to prepare a patch that sets the resolution of the PNM to the scan resolution. Similarly, and perhaps it is the same problem, it would be good if the preview showed 100% at the same size regardless of the scan resolution.
Having looked into this further, I think this may be a bug in the proprietary lexmark_nscan backend: it provides an option --scan-resolution, which is available in "Standard options", but (I am guessing) not the standard "resolution" option. Therefore, xsane can't tell that it supports different resolutions. Feel free to close this bug, therefore.