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I think I have discovered the reason for the offset I'm seeing, and it is
quite odd.

If I do the calibration using the buttons the VNA Sharp software, then I
get the offset mentioned.

If I do a calibration on the actual device, the results match perfectly
with the 8753.

The second chart shows the calibration done on the software, note that the
red and blue traces are not aligned.
The third chart shows the calibration done on the actual device using the
touch screen menu, note that the red and blue traces are aligned perfectly.



I thought this might be due to the calibration reset function, but that
does not seem to affect the results I am getting.
I also thought it could be due to me doing only the Open Short and Load and
ignoring the Thru and Isol. But that too didn't make any difference.

So I suspect there is a bug in the NanoVNASharp software, and that is
causing this issue.

Can anyone else replicate this to make sure its not just something dumb I
am doing?
You just need to do a cal on the device using the menu, and then measure
something close in value to what I did - something close to a 6dB
attenuator.
Then do a cal using the buttons on the NanoVNASharp software and measure
the same attenuator at the same frequency. I kept the freq below 300MHz to
minimise the noise and drift.

Roger