Perhaps you're referring to the division of a neutral army before a battle? If so, the number of divided units will be influenced by the strength of your army – the larger it is, the higher the probability that the neutrals will group into a single unit. And the weaker your army is (especially in the early stages of the game), the higher the probability that the neutrals will split into 3-4 groups. As for the archers/mages targeting, the decisive factor here is the **potential** damage (both from your archers and the neutral units) – the AI assesses the consequences of each move and, based on that, decides whether to attack the advanced "decoys" or strike at your snipers.