Another quest was released (Myths of the White Lands IIRC) which then had conditional logic for whether or not you'd completed Sheep Shearer (essentially sharing quest points).
The reworked quests replaced with entirely new ones work similarly (eg if you completed Black Knights Fortress you don't get more quest points from completing The Death of Chivalry.) And TDoC consequently had to have the same disproportionately high 3 quest points of BKF it replaced.
This gets even more complicated when some quests that have eg Defence reqs can be reset by Nastroth and replayed.
TL;DR quest points are a growing pile of tech debt that we could really do without. Fortunately we're gradually digging it down as opportunities to do adjacent work go out (eg as of the latest achievement system rework, quest cape is now a list of required quests rather than current quest points).