Using Bash, mysteriously don't need "export" command

I'm using Cygwin to get Linux functionality on Windows 11. Specifically, I am using Cygwin's xpdf to browse PDFs. It can open a hyperlinked URL if $BROWSER is set. Mysteriously, I found that I don't need to issue export BROWSER=firefox before issuing xpdf MyFile.pdf &. I just need a preceding BROWSER=firefox before the xpdf command. # This works BROWSER=firefox xpdf MyFile.pdf & Why is it that xpdf sees BROWSER=firefox even though I didn't export this at the Bash command line?