Each day it doesn't see your army it has a chance of forgetting where it is. For me this sounds awfully similar to how a player would remember roughly where an army is.
I don't know about you, but when I recall that there were units in the fog of war, I am nevertheless incapable of seeing exactly where they will go even when they change movement orders in the fog. And if a human player were to chase my army through his territory, I would not be able to adjust for his changes in direction while in fog, even if I knew an army is there.
If I were to do something like map hack, I could react that way. But that would be cheating.
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