Nope, that's not quite how it works. You have to remember, I do this by looking at the QSD, and how it should work.
What happens, is you get quest 805 from Mildun. If you kill mob 272 after that, the client runs it's little quest code.
It checks the conditions for 805-31, which are have quest 805, check percent (0 - 3), check itemcount 13098 < 1.
If these are met, it selects the quest, removes the item, updates a quest var [0] = 1
Now, since the first conditions aren't met, and this is a drop-through quest, the next kill will drop to the next trigger.
It checks quest 805, quest var [0] == 1, queat var [1] < 15
If these are met, it selects the quest, adds to to quest var [1]
Sooner or later this will be false (When quest var [1] = 15) and it drops to the next block
It checks quest 805, checks percent (0 - 7), check itemcount 13099 < 1, check quest var [1] >= 15
And it keeps going like that until the end. You can use the QSD editor (Rose Online Workshop) to look at QN-272.qsd.
Check percent is pretty much doing
check = random(0, 100);
if (check >= lowBound && check <= highBound) return true;
else return false;
But that's all done client-side.