At the risk of stirring the hornet's nest further, I think that AM5 is designed and sustained editorially as something of a sandbox game, and one of the principles of a sandbox is that the GM has everything figured out ahead of time and the players can do whatever they want, and go wherever they want, and whatever they encounter will be what is "actually there," by which they mean, "what the GM knew was there before we decided to come."
This is, I think, in contrast to a lot of more free form or story-based gaming where, for example, the GM changes things as the players are moving through the story because everyone at the table agrees its more fun that way. Tim's example of a dragon which suddenly develops acid blood is a classic example of this. It's not done to screw over the players, it's done because, hey, we have been building up to this dragon fight for three hours now, and we want it to be exciting. What is written down on the dragon's character sheet is, honestly, not especially sacred.
There are a lot of games today that do this, and there's a long tradition of it, from the recent Fate to classics like Amber Diceless and plenty in between. Every table is going to have a different tolerance and enjoyment level, but for my part, I very much enjoy both ends of the scale. And although AM is officially very much a sandbox, in which a GM can know a bewildering amount of detail about the setting before the players even make characters, it can absolutely be run -- and can be enormous fun -- as a more free form experience where no one even rolls dice, and where the GM and players are all making shit up as they go along.