Graendal would never expect Rand to simply balefire her whole palace out of existence. When Rand sent his messenger, Graendal figured that Rand wanted to "play", while he actually just planned to use brute force.

This is the point of Rand's analogy of two people sitting on a table, one thinks the game starts, but the other just punches the opponent in the face.

Any other interpretation, namely that Graendal outsmarted Rand, while he was trying to outsmart her, gives the Forsaken too much credit and the Dragon Reborn too less credit.