Originally Posted by mot29
Interesting idea, but I don't think that is the reason. There is a CHI and ORD and MDW are often no common fared.
ORD and MDW fall under the same fare. It's just that, sometimes, there are flight applications that don't allow MDW. It could also be that flight schedules only allow for one airport. Finally, it could be availability that is different, causing different prices. ORD and MDW both fall under CHI. Fares are for XXX to CHI.
Originally Posted by mot29
And ITA/Orbitz offers DCA, IAD, and BWI flights for WAS and the BWI fares are often lower. So I don't think that is the explanation.
ITA/Orbitz searching BWI when WAS is queried doesn't mean that BWI belongs to WAS. WAS only includes DCA and IAD. Fares are different to/from BWI than to/from WAS.
Originally Posted by mot29
And LON gets you LHR, LGW, and LCY (not sure about STN and LUT) and these are often not common fared.
LON includes LHR, LGW, and LCY are "common fared". One may see different prices simply because there's a lack of availability or an airline does not offer services there (e.g. United does offer services LGW). Also, taxes are different between LHR, LGW and LCY, but the fares are the same.