A bit of round and round on this one and the situation is as follows:
EPM gets its extension status directly from Asterisk. For extensions registered local to the PBX, Asterisk reports the local IP address of each endpoint. For extensions registered across the internet, Asterisk only reports the external IP. If you have multiple endpoints registered from a single external IP (i.e. all behind the same router) then Asterisk does not differentiate which endpoint is registered to which. This is true for PJSIP in Asterisk 13, it is possible that things might change in the future.
Long story short, the status reporting of PJSIP extensions in Endpoint Manager is as good as it is going to get, at least in the near term.