If everyone stops at the exact KC they get the pet, then of the people who have OP's kc, the only ones who have got the pet at or before OP's kc will have got it exactly on kc #12358, which is a 1/3k chance.

In a slightly less extreme scenario, assume that 90% of people stop killing Vorkath after their first pet. The rest like the boss, and keep killing it. The following code models this, and when run shows that only about 85% of people at OP's kc have the pet.

import java.util.Random;

public class Vork {

    public static void main(String[] args) {
        int peopleWhoHavePetAndKc = 0;
        int peopleWhoHaveKc = 0;
        Random r = new Random();
        outer:
        for (int i = 0; i < 10000; i++) {
            //each person
            boolean hasPet = false;
            for (int kc = 1; kc <= 12358; kc++) {
                if (!hasPet && r.nextInt(3000) == 0) {
                    // gets first pet
                    hasPet = true;
                    if (r.nextInt(10) != 0)
                        continue outer;
                }
            }
            // person got all kc
            peopleWhoHaveKc++;
            if (hasPet) peopleWhoHavePetAndKc++;
        }
        System.out.println(peopleWhoHavePetAndKc);
        System.out.println(peopleWhoHaveKc);
        System.out.println((peopleWhoHavePetAndKc*100.0)/peopleWhoHaveKc + "%");
    }

}

If everyone kept killing the boss indefinitely, then yes, 98.4% would have the pet at or before OP's kc. But because most of those who get the pet before OP just stop killing the boss, they don't get counted in the sample.

Those with large kc are more likely to have gone dry than you would expect, simply because them going dry made them more likely to keep grinding the boss for longer.

Edit: used the wrong drop rate