Not entirely sure what you are saying but some quests are to defeat a certain number of enemies and other quests are to collect a certain number of items from an enemy.
On the first one, the number of enemies you defeat will match one for one. Meaning if you have to defeat 4 enemies, you only have to defeat 4 enemies.
On the second one, you won't always collect the item. So you may defeat 3 enemies and only collect one of the necessary items.
So in Krok, the first time around you are only defeating a number of spiders. For a later quest you are collecting spinners so the mander can make time thread (or something like that). On the second one it may take you 6, 7 or even 20 spiders to collect what you need.
It's annoying but happens a lot throughout the game. Best thing to do is go to a busy realm and find someone else who is also collecting items and battle together. Have fun!
Beyond the fact that it is a 'collect quest' as outlined above, you also have to understand that as your character matures in the game, the drop percentage drops. While for the spiders you usually get one every combat, by the time you get to CL you have to defeat two dozen plus foes (these MOBs are about 2000 health each) to collect three things.
Needle in a haystack? CL designers must have had that in mind.