Well, firing normal arrows means you hit the people carrying the ram more often and more deadly, so the ram's pushing speed is reduced. That gives the tower more opportunity to shoot the ram and its crew too.

Still, I'd expect 5 flame archers to be more effective.

Something else to consider is that in my campaign games, my ram catches fire much more often against a large town (large wood) with 2-3 towers firing at it than it does against a stone castle northern (4 towers firing at it at further distance already and at higher altitude) or even a huge stone wall (2-3 towers, high altitude)

Maybe large town towers are best :p