Think of them as CIA analysts or TV pundits.
Some prophecies were ambivalent, like saying if a king invaded his enemy a great empire would fall, but the invader forgot to ask which empire, and he lost.
But other prophecies sound like they had intelligence and military knowledge behind their prophecy.
And when Athens was in danger from a Persian invasion, the first prophecy told them to flee they were doomed.. which happened, and it didn't take a prophet to for see that.
But when the Athenians demanded a second prophecy, they were told to rely on their wooden walls, which was interpreted by the Athenians as their ships. I always figured they were influenced to make that one up by someone who knew that there were too many Persians to live off the land, so figured they could block his supply line.