(2-part post):
Have you read the small print?
(In abbreviated form):
"The data displayed in this web site is NOT a display of AVAILABLE flights but is a database of prices and should be used as a means of researching the best fares for a given journey on a date or range of dates. Once a suitable fare has been found, it will be necessary to contact the travel agent to check if seats are available on the date you wish to travel. All airlines sell only a limited number of seats at the lowest fare on each flight. When these have been sold, you may be offered a seat at a higher fare. However, the price will NOT be removed from the fares database as the cheaper seats may still be available on other dates or subsequently become available again due to the airlines releasing more seats at the lowest fares due to fluctuations of supply and demand.
In some cases, certain fares are only available to particular types of passengers. Examples of this are student and youth fares, OAP fares, group fares and inclusive tour fares requiring accommodation elements"