The impression of airfare aggregator sites is that they're mechanical purveyors of ticket information, but this is not always the case, writes Airfarewatchdog. Some sites won't list some airlines, either permanently or temporarily, and for no reason transparent to the consumer. Case in point:

UPDATE: For most of these carriers, this doesn't have to do with impartiality, but rather the difference between two different kinds of carriers. See reader badgeman's comment.

Orbitz: Sun Country, Jet Blue, Southwest, SkyBus, Allegiant, USA3000
Travelocity: Southwest, SkyBus, Allegiant, Virgin America
Kayak: Southwest, SkyBus, Allegiant, USA3000
Expedia: Southwest, SkyBus, Allegiant, Virgin America, Spirit, USA3000
Sidestep: Southwest, SkyBus, Allegiant, Virgin America, USA3000

This is why it's good to check out your routes on multiple sites, we like farecompare and farecast. Notes airfarewatchdog, "many...airlines reserve their best fares for their own web sites, even if they do list most of their fares with a third party site."

Which booking sites exclude which airlines? [Airfarewatchdog]
(Photo: Getty)