It is possible that a person is just pretending to be gambling for fun but instead, they aim to win but of course we cannot tell whether the person is actually gambling to have financial gain because we can't read peoples mind. There are also instances when a person is truly betting to have fun but with frequent exposure and taste of financial gain, he might change his stance to betting to have financial gain from betting just for fun. 

In worst scenario, a person who just plans to gamble for fun may end up addicted to gamblings because of different factor that leads him to lose his self-control and trap in a situation where he can't control his urge to gamble.  It is said that as long as a person is exposed to gambling, the possibility of being addicted is always there.  So yeah it is possible for someone who claims he is gambling for fun to get addicted to the game.