Critical hit rate.
Raccoons are a very bad target to test this on because they might not be capable of scoring critical hits at all, with their stats and skills the way that they are. In other words, I don't think they have a Rank F Critical Hit.
Which agrees with what I described in my 6th paragraph.
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.