It is RNG, but your chances are very dependant on the Gambling skill.
You need a relatively high Gambling skill for poker (even at 70 you will be loosing a few, but the reward is worth it). Two of the Academy classes raise it.
It also increases if you gamble at the tavern or the gambling spot in the area further from Watery Eyes. You can increase it even by loosing in poker but it can be expensive if you're not winning at least some.
Also, poker usually recquires a Hand above 800, so if you are below 500 it is better to take the high risk option,