Well, for one thing they can't see the future completely and some are better at it then others (does this have to do with the nature of their Shards, or is it a matter of personal practice on the part of the Shardholder?)
Also, it's highly possible that they can't predict the actions of other Shardholders directly (although they may anticipate by other means) because it would cause a similar result to two people burning atium.