Origin Form Dialga VStar - Weakness not taking double damage?

I played a match using Charizard EX deck, and the opponent used Dialga VStar. The opponent took 2 prize cards, so burning darkness base damage would go up from 180 -> 240. The Origin Form Dialga VStar shows a weakness of fire, but it only did the base damage of 240. Is there something wrong with the weakness to fire in this case?