Because in the local/private network are many hosts, like your phone, pc and your server. Exposing means that the device that is exposed gets basically everything forwarded what usually the router would handle. Exposing does expose a host to the WAN.
Forwarding a Port only forwards the specified ports. You can use multiple hosts for that. For example you can port forward port 80 to your Phone to port 321 or whatever and port 443 to your server at port 20.
So for port forwarding you need the port on the router the host and the port the traffic should get forwarded to on the selected host.
So you are saying, when exposing a host then the host is reachable over internet but when using port forwarding it is not?
How you check this? What commands/procedure?