I think something is different about getting the other caps, i mean the NPC by the eggs offer only the quest for Pristine Egg Cap (rank 5 quest) it is specifically detailed in quest journal so it's probably impossible to get any other cap through that quest. So somewhere, someplace there must be a trigger to unlock second (and so one) quests that will give you different means of obtaining eggs and that there will be specifically shown that the reward is different cap than the Pristine One.