I love the way people jump onto a web site and get a snap of fares then trumpet them as being despicably expensive!
As with any carrier, dependant upon the load, the forward booking advance time and the time of year then the prices change. Blocks of seats are allocated specific booking prices dependant upon how far in advance of the flight date they are booked. The increase of fares is generally symptomatic of the fact that the 'cheaper' seats are all booked and no longer available with that carrier. That may not be the case with another carrier who still might have discounted seats available dependent upon their costing structure.
Never snapshot prices! Have a look at the LoCo's and then try and see if you can still get that £0.99 seat the day before you want to travel. Simple answer, not a hope.