I had also noticed this.
I wonder if it has less to do with actual firepower, and more to do with sustainability of play. As the battle wears on the 74s broadside erodes faster than the First Rater and goes on further reflecting the extra guns. This prevents the knocking out of the 74 with a single punch. As we can see from many battles 74s often gave a good account over larger ships although they inevitably ended up as hulks by the end of the battle.
That is the only reason that I can put forward.
Rob.