Or... What if women perceive the world different when tilting their head, than men, so when they see a tilted glass drawing, they might not consider the glass as tilted, but rather the drawing as tilted, which causes them to perceve the gravity in the draiwng as tilted as well.
It would be interesing to see a similar study comparing how women and men read text which is tlted 90 degrees: If they rotate their head, or if they simply read it the way it is presented.
Well that's a bit 'out there' :D