Port 27015 closed but my server works?

When I do a port checker it always says port 27015 is closed, yet I can get people to join my server on port 27015 using an external ip.
So they type 209.xxx.xxx.xxx:27015 (the external ip) in gmod console and it works fine . Does anyone know why or how? I am port forwarded as well.

port checkers aren’t exactly accurate