Seems strange. If you plug your ticket number into the Saudia website, you should be able to see the new fare basis if you were indeed re-fared in F. I'm trying to construct a scenario where inventory changed and allowed this to happen, but I'm coming up empty. Unless the fare chart changed while you booked? But that seems unlikely.
ed. - yeah, the only fare booking into F that I can find is FUA (the ~$1,200 full F fare). I'd be curious to know what the ticket lists as the basis.