DPI inconsistency

[Yuri]

My NVidia card has the resolution 3840x2160.

xorg.conf has this section, following the recommendation from 
https://linuxreviews.org/HOWTO_set_DPI_in_Xorg:

Section "Monitor"
         Identifier   "Monitor0"
         VendorName   "Monitor Vendor"
         ModelName    "Monitor Model"
         DisplaySize  630 340    # In millimeters - setting for DPI
EndSection


Xorg.0.log has:

 > [ 37714.549] (--) NVIDIA(0): DPI set to (159, 156); computed from 
"UseEdidDpi" X config


Based on the DPI computation for resolution 3840x2160 and size 630mm x 
340mm - DPI should be 155x161.

This is close to what Xorg.0.log has, but is not equal to it.


But xdpyinfo shows 96x96:

 > xdpyinfo | grep -B 2 resolution
 > screen #0:
 >   dimensions:    3840x2160 pixels (1016x572 millimeters)
 >   resolution:    96x96 dots per inch


Why doesn't the xorg.conf setting of DisplaySize translate into the 
actual DPI value in xorg that xdpyinfo shows?


Thank you,

Yuri