"The painting's quite obviously a fake." Molly felt like saying to Sherlock. She had worked it out before him. But telling his this would be altering the game; the winner couldn't be pushed into winning, he had to win by himself. There was no reason for this game other than Jim liked to play with Sherlock and Sherlock was bored so he let himself be played with.
So instead she pretended to be in awe of his amazing deductive powers; they were no better than Jim's, or even her's, really. Mycroft's were pretty good, too. Sherlock just didn't realize that he wasn't the only person in the world with deductive reasoning.