The whole secret behind fortune tellers is that they are perceptive and they use clues to play the percentages. There are just little things that they might notice about you to give them a hint about what you might think would be relevant to your life to hear.
Also, most of their statements are really GENERAL that could technically apply to a lot of people, so those that go to the fortune tellers always get a weird feeling like they really connected.
Kristina