Critical hit rate.
Just spent some time mastering Rank 1 Counter Punch on my human. I started on the assumption that the critical hits required for skill training were determined by my character's chance of scoring a hit on the target. Started with the Dire Wolves near Ciar Dungeon since they were weak and will usually attack promptly when provoked. Less than 1 in 10 counterpunches were scoring as critical.
Then found some comments suggesting that the Counter Punch critical hits were actually related to the targets chance of scoring a critical hit on my character. Took off all armor, reducing protection to zero. Moved to hard mode Dorren's Request, green switch for crag cows and shadow shires. Got a significant increase in frequency of criticals for training. Small sample, but it's plausible that I was getting 30%, which is often mentioned as a cap.
So it looks to me that the criticals are being determined by the target's critical chance rather than the character's critical chance. In general, this would be odd for a character skill, but for a "counter" skill I guess it's plausible.
But then I can't explain your Raccoon example. So I tried to verify that.
Some MMORPGs compensate for poor AI by having asymetric combat formula. That is, the formula for calculating combat outcomes are different for players and monsters. We think the formula for calculating a player's character's chance of scoring a critical hit is (Attacker's Critical %) - (Target's Effective Protection * 2). Do we have any source suggesting the same formula is used for calculating a monster's chance of scoring a critical hit on a player's character? So, I was wondering whether the Raccoons are scoring critical hits due to the monsters having a different formula.
My giant has 45 protection. I just used Enthralling Performance to get a bunch of raccoon's to attack it repeatedly. They only do 1 damage. So how do I know if they're scoring any critical hits? The words "Critical Hit" only appear in the middle of the screen when a player scores a critical hit, not when a monster scores a critical hit on a player. So switched to my human, which hasn't yet fully trained the critical hit skill, so if it receives a critical hit I get a notification that the training count for critical hit increased. No - those raccoons are not scoring any critical hits. Even with armor removed, reducing protection to 1, they aren't scoring critical hits. Switched to my elf, that hasn't yet fully trained Counter Punch. Yes - it can get criticals when using Counter Punch on Raccoons. No - it's not receiving any critical hits from basic attacks from the Raccoons.
So I think I've verified your argument.
Which leaves me confused. Raccoons aren't scoring critical hits on normal attacks, but I can get increments to the critical hit training component of Counter Punch with raccoons. That suggests the target's chance of critical hit is irrelevant. But if I do things to increase the chance that the target scores a critical hit on my character, like removing all armor and fighting harder monsters, the frequency of incrementing the critical training component of Counter Punch increases a lot. That suggests the target's chance of scoring a critical hit is relevant.
So I don't know what's going on. Does Counter Punch use a formula for calculating chance of critical hit which is inconsistent with very other skill in the game? I'm out of ideas. Giving up for now.
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.