airlines fight each other with lower fares from competitors hubs.

take united and delta

united's main hub is Houston
delta's hub is atlanta

delta will start set trippin and list $100 fare Houston - NYC (but a stop over in atlanta)
but that same houston-atlanta fare on delta will be like $250 if you bought ONLY THAT PORTION

then united will launch a $100 fare atlanta-los angeles fare but via houston
that same atlanta-houston fare will be $250 if you only bought that portion on united.

basically these are "loss leader" air fares that they are trying to steal marketshare from competitors at all cost.

there are huge mathematical algorithms that go into pricing airline flights all the way down to the cost of a barrel of jet fuel and the amount (poundage) of luggage each passenger checks in.
commercials airlines make a considerable amount of revenue by transporting mail/parcels for the post office and other shippers

****
pink toes: http://i.imgur.com/WN7DPL1