320JI
I also use Ryanair and had noticed what you describe. I think they can make the taxes/fees amount to any figure they want, which gives rise to the anomaly you have observed. We just have to look at the final total cost of the flight and decide if it is a price we are prepared to pay.
As I often travel from Luton to Murcia and back for about the same price as a train ticket to London I am not complaining. I also find them very punctual and reliable.