I guess it works as a psychological moment to give the user the impression that he was so close to winning the main prize. However, I think it's normal for the gambling industry because many other gambling sites use a similar trick.
Its not a trick, it is called suspense. By adding suspense on a simple game like this, it excites people and makes the game a little bit more interesting. Imagine the WoF feature but with an instant result ( something like a dice game which gives you an instant result after each click ), it wont be be as interesting as the current WoF.
Every gambling game out there has a built in suspense onto it otherwise it would be pretty boring.