Could it possibly be something like this?
Let's use double crescent for example. At rank 6, the probability to knock down enemy is 18%.
Now instead of choosing 18 variables between 100 to make it work (which does nothing to prevent infinite knock down, or not having once at all in 1000+ hits), they use a Pseudo Random Probability Distribution.
Instead of having 18% chance to knock down for every attack, they make the first attack 3.645% chance to proc knock down. If the first hit didn't proc, then the second hit would be 7.29%. If the second one didn't proc either, third hit would be 10.935%. Adding 3.645% to each hit if the last one didn't proc a knock down. If a KD do occur, then that value is set back to initial, or 0%. In this way, the most hits you will hit is 27, before you get a sure knock down. Most likely you have already kd before it as it past 50% (around 13 hits).
Note: I am not sure about the percentile, the table was using another game's PRPD's conversion table, so it could be off. But this might be the reason why you can always get a knock down within a certain hits, and only at a few times would you be able to knock the boss back down right as it gets up.
If you want, here's the
source. But don't wander in there if you don't want some intense reading.
Edit: Of course, in Vindictus, the boss would have a knock down resistance, and it should affect negatively against the kd count, and you will have to do more hits on higher boss to get them knocked down.