That's related to the interesting philosophical thingy mentioned earlier. Since we're dealing with a make-believe world, things aren't "real" until the DM tells the players that they are.
When does the world's reality become set? Say the DM had a plan for the solution for the mystery when doing his session prep, but then thought of a better one on the drive over to the session, so he changes it. Is that different from changing the plan when hearing a better idea from a player? I don't think there's any practical difference.