I am not sure how flawed the system is but by the sounds of it they prefer to call people who recently applied.
I can defenitely see the logic there since there is a pretty big chance that someone found other employment in those 5 years, so the chance of someone accepting an interview is a lot bigger when calling people who recently applied.
looking at the avarage response time and numbers of interview spots available weekly, it looks like they are reaching the bottom of the database. That is why they might be calling people who are in the database for a lot longer.
Ryanair is planning to hire another 1000 pilots this year alone, primarily cadets. Interesting times...