Why is the price I pay for a certain airline ticket not always the same?

Any one flight could possibly have anything up to 30 or 40 fares available, all with varying lead in times and ranging from fully flexible to non changeable / non refundable. This is in order for the airline to maximise its yield on each flight.

Each fare will have limited availability and naturally the cheapest fares will go first. So at any one moment in time the last fare of a certain type can be taken, thus reducing choice.