You could have multiple hosts on the internal network appear to the outside world as the same (IP, TCP Port) tuple because they are connected to external differ

You could have multiple hosts on the internal network appear to the outside world as the same (IP, TCP Port) tuple because they are connected to external different addresses. To illustrate, let's say my network's external IP address is X. Let's choose some port number Y. A packet addressed to X:Y could be going to any one of several machines on my internal network, because the NAT uses the source (not just the destination) as part of its lookup. So X:Y does not uniquely identify a machine... it's only part of the total address.