John,
On my NanoVNA if I have my cables attached as your photo shows; and set the frequency range to 50k-900MHz, performing a reset gives me a display with the reference level at -10 dB. When I perform a "THRU-DONE-BACK-BACK" sequence, my new reference level is 0 dB. If I narrow my range to 50k-5MHz as shown in your photo, my reference level stays at 0 dB and does not drop 10 dB as your photo shows.
