I think it just represents that the longswords were quite lethal, usually swung with great power and thus able to shatter shields and cause damage through helmets and armour. Most of the units that use longswords lack armour (defence rating), aren't usually particularly disciplined and - unless their initially lethal attacks break the enemy will pretty rapidly lose morale. So, rather than being "skewed" in favour of the barbarion factions, it is a reasonable interpretation of the era, I think.