I believe the higher bar represents the inscription that was on the cross (the inscription saying "King of the Jews", being what the non-Jews who physically carried out the act called Him in place of the term "Messiah"), and this inscription was on a shorter, higher board than the board that they pinned His hands to. While I may not remember it correctly, the lower one, meanwhile, is the board where God's feet were, and it symbolically slants upward towards the penitent thief, who was on Christ's right. If only Spas or Aedra were still here to confirm that I'm right on this.