Tripair is a Greek TA (Part of altair/Petas.gr). Chances are that they got a special deal or fare with Aegean. I'm pretty sure I have been sold a seamans fare through such TA in the past (though I think that was the now defunct TravelPlanet24). You may/could also be on a group ticket or something similar.
Or...it may just be that they pre-purchased a bunch of seats in P while those fares where available, and sell these to you right now.