IPsec behind router without port forwarding

I have a IPsec N2N connection between 2 IPFire. One of them is behind a FritzBox. The Fritzbox is not working as exposed host and no port forwarding is set up, but the connection still works. Does anyone have an explanation for this?

Thanks in advance.
Krause

I have.
Not exposed host “calls” exposed host. Exposed hosts can receive and respond.
NAT keep answering to not exposed host.