There are better experts than me out there on fare structures of low cost carriers. However basically depending on route - aircraft type - popularity - (in particular low yield days of travel - such as mid week and not weekend) more seats would be available at a low cost. Other dependables are time of year - and the competition on routes. The up take is monitored constantly by the airlines and price adjusted by increases fares or reducing appropriately. This in general means that the closer you get to departure dates the price is higher - however on ocassion can be lowered significantly if seats are failing to be sold. Most carriers would never go below their "lowest norm" price except in circumstances of promoting routes or on special ocassions etc.

Nivsy

I have noticed that when booking flights with Air Malta on the internet it sometimes pays to book two independant seats (two bookings) rather than two together (one booking). This is because they have one seat left in the cheaper priced bracket.
We always check in together and obtain our sets next to each other but one of the seats is around £40.00 cheaper!

The simplest structure is a certain amount of seats at each price. For example say they have 8 seats at each price, so every 8 bookings the price goes up.

However it is not as simple as that - airlines use very very sophisticated systems to manage yields. Like nivsy said, they calculate flight demand, i.e. midweek, weekend, morning evening - if a flight is low demand it will have more cheaper seats. Also, as it becomes nearer the date of the flight the price can increase.

Some airlines systems continually monitor demand. If a particular flight gets a lot of booking inquiries on the site, then the price will go up as the system will think there is high demand. So if your thinking of booking a flight don't keep going back and quoting it over and over as you may push the price up!

In simple terms, your Mallorca flight probably has high demand or is now fairly full, whereas your Murcia probably isnt filling up as quick or in demand as much.