The word 'from' being the key word. I don't know the rules but I guess a certain percentage of all flights sold must be flogged at this price. So say they are offering £9.99 from LBA to AGP - it doesn't mean every seat will be that price.
Ryanair are the most extravagant for this - not the worst, but the biggest difference.
They offer fares as low as 1p yet if you book the day before it can be over £200 before tax!