Well that's interesting.
I was wondering where do you put your 16 peltasts? It is hard to imagine 16 peltasts in a static defensive posture contributing much to the destruction of 2,000 units. I'd like to see a screenie of that. Were they just bait, or doing helpful battle?
As for the 2,000 defeated Pontus units, were they perhaps shot up (wounded) by towers/walls on the way in... or maybe they were peasants. What was their compostion? Did they all make a frontal assualt, and take tickets to wait in line whilst their buddies were slaughtered in front of them (it would be impolite to get killed out of turn)?
The amazing thing is really that the hastati were not eroded (killed) slowly as the wave after wave of 2,000 attackers came in... this is because the Hastati do not fight at a distance at the end of a spear (like phalanx), and will take casualties when they come to grips with an enemy. The triple formation, if sucessfully created, will create a tough wall and rapid deaths of those that approach, but those that approach should inflict damage, and that should cause the Hastati to die, even if they achive a high kill ratio.
Even a screenie at the end of the battle, with the Hastati in place and the bodies in front might reveal if there is a game bug going on.
Have you been able to duplicate the battle, and have the same results?