Critical hit rate.
Its possible, since I haven't worked with the skill much, that the Additional Critical Damage is actually Additional Critical Chance. That's how regular Counterattack works after all. That's also how Lance Counter works, so I find the "Critical Damage" to be very odd. If you get a chance, try to see if the rate of Criticals from Raccoons seem to match the rate the skill's "Critical Damage" is supposed to be at.