onion, I think that sort of thing happens a lot - I recently enquired at a city centre long haul specialist travel agent about flights to New York for July and was immediately given prices from LHR instead of BHX, despite there being a daily UA BHX-EWR flight. To be fair, LHR flights were cheaper than UA from BHX so that's probably why they were offered first but you would have thought they would have looked at BHX first and then LHR.
Obviously it's frustrating and those who are not so route knowledgeable will go for the first flight they are offered and assume that a flight to the same destination is not available from their local airport. Maybe there were big differences in prices between MME and NCL when you enquired, to be honest I would have thought NCL flights were probably cheaper?