Shell script executes without shebang in sh

In my local and in some of the labs, shell script executes without the shebang statements in Bourne Shell(sh).
I verified running echo $0 present working shell (since echo $SHELL was showing me login shell) that working shell is sh, can someone please explain why?

When you do not have any #! (called shebang ) line, sh is used. To check that, you can run the following script.

ps -p $
echo -n "The real shell is: "
realpath /proc/$/exe