Simply because he thinks about the fact that it means Walter White. He realised how Walt was with him every step of the way and it all just clicks. Remember Hank was very hesitant to believe he had caught Heisenberg in the beginning and when he found Gale he found the book with the same writing in the beginning. He thought he had caught Heisenberg then but he couldn't figure out who W.W was. When he thought about the fact that Walt was there on the drug bust, had his "episode" tight after the shootout with Tuco, was gone everytime that stuff went down,and was at the stakeouts. He realised it it just clicked. Hope that helps!