Qatarisun ... This happens in most cases, Air fares are published and decided by IATA ( international Air Transports Asociation) which is called the published fare. Then dpending on the market airlines and the civil aviation decides a fare strecture which suites the market which will be in use. This can vary always....

Apart from this, Airlines undercut their fares to fight the competition..the ticket would show the market fare but will be sold on a much lower rate, sometimes the agents keep them as their retention, sometimes a portion will be diluted and parted to the passenger.

These fare differnces were much more before and now is been balanced to a gr8 extent after the intro of E- ticketing.

Its better to always compare both the origin and destination on fares available, there might be even special offers if they are goin thru a lean period.
cheers