If 3 minions attack a single one, while on the other side, 2 minions attack the same one, and the other one attacks a diffirent one. 1 minion takes 3x the damage, and the enemy one just 2x. This makes 1 minion die 50% faster.
And this is very much possible when the attacking force is archers and defensive is melee. As then the 3rd melee of the winning side, is now also causing extra damage towards the other side, causing a 3v2 situation on their side. This 3rd minion gets a delayed death, but already applied a few extra attacks. Which is enough for lanchesters law to appear.
And yes, its perfectly possible to happen by randomness! Because the numbers are this low, and because the melee side is only used as a buffer which protects the archers (the higher damage of the group).