Or if there's another predictamancer working for another side. Then there can be logical contradictions.
Example:
Side A is defending five cities, and has enough troops to defend two of them from side B's force.
Side A uses a predictamancer to determine which city/ies side B will attack the heavies. They then put most of their troops in those cities.
Side B uses a predictamancer to determine which city A will defend the least, and attack that city.