There must be some aerodynamic lift under board; perhaps not between hull and water but certainly under hull out of water. Perhaps when rig is unloaded this lift is allowed to have an effect. Afterall even in only 15 knots of wind apparent wind hitting hull will be at least equal to board speed (probably more?) I`d have to look at vectors but I`d suspect in 15 knots wind with 20 knots boardspeed (easily achievable on Formula) there must be at least 22 knots apparent wind hiiting hull. That would certainly have a lifting effect.
And turning into wind would initially increase apparent wind !!