You do realise that the accuracy vs. towers is different to the accuracy vs. units? Don't know if that would make a difference.
I didn't know that. I've just tried to contrive a Custom Battle with ballistae vs infantry where the infantry just stand there and get shot to bits (as can happen in some campaign battles), but it's proving to be more difficult than I thought.

I tried a couple of river crossings. One against all slow moving halberdiers - they simply advanced inexorably at snails pace across the river and knacked all my ballista crews. Another against all peasant archers with a big wad of my own armoured swordsmen stuck in the middle of the bridge for them to shoot at while my ballistae picked them off from a distance. They too bum-rushed across the bridge in a mass butter knife charge.

Perhaps in a siege defence sally the enemy might comply and stay put. Does anyone have any ideas on testing arty accuracy vs units?
Also, ballistas are pretty accurate to start with, so a small increase might not be very noticeable.
Honestly, there really was very little variation in the hit rate of the ballista units. Sometimes the greenhorns even crept ahead. All towers were destroyed at about the same time though, so any variation was averaged out. I tried the battle again a couple of times to make sure, with similar results.