The effect of "Ancient Sacred Wyvern" increases or decreases its ATK based on the current difference between your and your opponents' Life Points. The increase or decrease is NOT based solely on your opponent's Life Points. If either player's Life Points changes, then the increase or decrease changes accordingly.
Example: If you have 2000 Life Points and your opponent has 1000, then "Ancient Sacred Wyvern" gains +1000 ATK. If your opponent's Life Points then increase to 1500, then the ATK increase to "Ancient Sacred Wyvern" becomes +500.