it probably does come down to the better attack, having an extra point for both the attack roll and damage. Also consider the XP value, not all minions are created equal. We may want to run the numbers to confirm, but try scaling up the numbers to simulate a pretty solid swarm.
Lets say a 400 XP encounter.
Rotters cost 38 experience, 10 zombie rotters (10.5 or so dropping the fraction)
Cutters cost 25 experience, so 16 cutters.
(at this point, since I had assumed cutters were more expensive, it appears I am not making a great case for the cutters).
Assuming all targets are attacking AC 15 targets: 55% of the rotters will hit, so 5 will hit and inflict 25 damage per round on average.
For the cutters, 50% will hit, so 8 rotters doing 4 damage, 32 damage per round.
Yeah, at a glance the Rotters are probably broken in some way. All they can do is move and hit, and their defenses are weak. Could be that they had some additional ability, perhaps the grab, that was removed after testing, but the rest of the creature was not improved. Alternately, it could be a case of the formula determining a monsters level putting weight behind the immunity to disease and poison.
But when a level 3 minion is only able to do melee attacks, and a level 1 minion has an attack, and 2 abilitys (+1 damage with combat advantage and the goblin tactics ability to shift when an attack misses), something is wrong here.
END COMMUNICATION
Low defense for high damage makes some sense for a regular monster, but when you only have 1 hp... Any situation where you could leverage the zombie rotter's immunities and darkvision to make them a threat would apply equally to decrepit skeletons - who have better defenses, more speed, same melee attack bonus, 1 less damage, and a ranged attack... and are two levels lower.
