My theory is that the reservations systems (see the continental.com quote above) all pull their data from their respective GDS (Global Distribution System, NW uses WorldSpan) and those sometimes calculate the city code distances, so MSP-HOU. Then, on top of that, the figures themselves may be -/+ a few %. I guess that's why they say the mileage is "approximate" from the nwa.com reservations.

I don't think it's a function of them "not being able to figure it out yet" as much as it is a function of their reservations systems pulling the figures from their GDS rather than calculating it from the great circle route. Plus, nwa.com reservations is powered by Orbitz, so it may be an Orbitz or other supplier issue (i.e. WorldSpan).