nmap inconsistent results - via intermedite router?

I have the situation where I have 2 linux machines. Both are sarge and
both have nmap 3.81. One machine is on my home network, the other on
the internet.

When I run an nmap from my home machine to the internet machine, it
reports the 5190 (aol) port as open amongst the usual ssh etc. If I run
nmap from the internet machine itself (on its IP address, not loopback
etc), it does not have 5190 as open!

Having looked into this as much as I can, I think that nmap is picking
up a port from the home firewall (a DG834G, which is reported as having
this port specifically left open - amongst others).

But why does an outgoing request, targetted at a remote server then
come back with information from another source? How can I test the
server remotely if it doesn't bring back the right details?

Anyone any ideas? Apologies if this is not sufficient info. I need to
sleep and thought someone might be able to answer off the top of their