Nonsense Question: How Does Augury Work?

When prognostication or prophesy is used in a story how do you imagine it works? The future has a set outcome and the seer can see part of it, but he might not be able to see everything anymore than someone with a telescope can see everything all the way to the horizon. Fortunetelling somehow taps into the magical equivalent of the futures market or current weather conditions to predict a certain amount of the future. God/gods have a plan and are just using the poor prophet as a mouthpiece to announce their plans. The wizard doesn't actually know the future, he's performing a sort of magic to force the future to conform to his vision rather like someone deliberately crashing a market for personal gain. Just like in the real world he's being vague enough that he can claim credit later on. Other?

Or are signs and portents in a story a cheat and something no good writer should muck about with?