seers were like weak points that let out the pressure, when the right listener was nearby.
[...]
the seers themselves didn’t remember their prophecies, because the message wasn’t for them. And the messages would come out in riddles, and only someone who heard the prophecy in the seer’s original voice would hear all the meaning that was in the riddle.
MugaSofer comments on Harry Potter and the Methods of Rationality discussion thread, part 16, chapter 85