Why is this script treating the files with the same names as the same files?

#!/usr/bin/bash

install_wm() {
    echo "$(dirname "$0")"
    cd "$(dirname "$0")" && pwd
    mkdir -p /root/.config && cd /root/.config &&
    git clone https://git.suckless.org/dwm  && cd dwm && pwd &&
    diff "$(dirname "$0")/config.def.h" /root/.config/dwm
    cp -f "$(dirname "$0")/config.def.h" /root/.config/dwm &&
}

install_wm

When I run this script as root I get

.
/home/jim/CS/SoftwareDevelopment/MySoftware/Bash/ubuntu-server-LTS
Cloning into 'dwm'...
remote: Enumerating objects: 6504, done.
remote: Counting objects: 100% (6504/6504), done.
remote: Compressing objects: 100% (3216/3216), done.
remote: Total 6504 (delta 3733), reused 5933 (delta 3287), pack-reused 0
Receiving objects: 100% (6504/6504), 6.18 MiB | 8.86 MiB/s, done.
Resolving deltas: 100% (3733/3733), done.
/root/.config/dwm
cp: './config.def.h' and '/root/.config/dwm/config.def.h' are the same file

I run this script as root. $(dirname "$0")/config.def.h is my config file that has different content, and is located in the same directory as the script, than the one in the cloned repo. Why do I get from cp './config.def.h' and '/root/.config/dwm/config.def.h' are the same file if the files only have the same names and not the content? Also when I run diff on the two files manually, outside of the script, I get the output that shows the difference between them:

22,23c22,23
 static const char *tags[] = { "1", "2", "3", "4", "5", "6", "7", "8", "9" };
> 
30,34c30,31
 	{ "Gimp",     NULL,       NULL,       0,            1,           -1 },
> 	{ "Firefox",  NULL,       NULL,       1  /* commands */
66,71c61
 static const char *termcmd[]  = { "st", NULL };

whereas I don't get any output when diff is run from within my script. What is happening here?