I get same under Tk.pm and Tcl::Tk.pm

screen name     :0.0
pixel width     1024
pixel height    768
scaling         1.3330053300533
mm width        271
mm height       203

$ perl -MTcl::pTk::TkHijack pm11129491.pl

screen name     :0.0
pixel width     1024
pixel height    768
scaling         1.3330053300533
mm width        271
mm height       203
[download]

screenmmheigh tracks to https://iweb.dl.sourceforge.net/project/tcl/Tcl/8.6.11/tk8.6.11.1-src.tar.gz

$ ack -ri HeightMMOfScreen tk8.6.11 -B1
tk8.6.11\generic\tkCmds.c
843-    WidthMMOfScreen(screenPtr) = width;
844:    HeightMMOfScreen(screenPtr) = height;
--
1485-   Tcl_SetObjResult(interp,
1486:           Tcl_NewIntObj(HeightMMOfScreen(Tk_Screen(tkwin))));

tk8.6.11\unix\tkUnixEvent.c
201-    }
202:    if (HeightMMOfScreen(DefaultScreenOfDisplay(display)) <= 0) {
--
205-    mm = HeightOfScreen(DefaultScreenOfDisplay(display)) * (25.4 /
+ 75.0);
206:    HeightMMOfScreen(DefaultScreenOfDisplay(display)) = mm;

tk8.6.11\xlib\X11\Xlib.h
115-#define WidthMMOfScreen(s)  ((s)->mwidth)
116:#define HeightMMOfScreen(s) ((s)->mheight)
[download]

I recognize that 25.4micron = 0.001 inch = 0.0254 mm

#!/usr/bin/perl --
use strict;
use warnings;

use Wx();

my $name = Wx::Display->new->GetName;
my $dc =  Wx::ScreenDC->new;
my( $width, $height ) = $dc->GetSizeWH ;
my $scaling  = ' ??? ';
my $ppi = join ' x ', $dc->GetPPI->GetHeight, $dc->GetPPI->GetWidth;
my( $widthmm  ,  $heightmm ) = $dc->GetSizeMMWH();

print <<EOT;

screen name     $name
pixel width     $width 
pixel height    $height 
scaling         $scaling 
ppi             $ppi
mm width        $widthmm
mm height       $heightmm

EOT


__END__

screen name     \\.\DISPLAY1
pixel width     1024
pixel height    768
scaling          ???
ppi             96 x 96
mm width        280
mm height       210
[download]

The mm width/height seem close enough, the 7-9 mm difference could be the BLACK margin borders of my display. I recall adjusting margins on monitors years ago.

As best I can tell with a ruler , not counting the black borders, my mm height is closer to 205mm , so neither tk 203 nor wx 210 are exact