Already had that conversation BPF thanks. It has been questioned and reweighed which is why the last weight schedule was done, with the figures that it has. I'm assured that its been looked into as apparently the question of it's weight (or lack of) was a cause for concern.
Just back from shooting instrument approaches in it actually, two up, tanks full no probs.
I think it's just one of those weird ones, maybe it's made out of balsa...
By the way, where do you get your sample weight from?