Why did the community make the distinction between "selection" for Receiver of Memory and "Assignment" for all other occupations in The Giver?

"Assignment" is a common and relatively simple process in the society.  At the age of twelve, every citizen is "Assigned" to a job they will do for the good of the community.  Children are carefully observed during their formative years, and based upon the talents and interests they demonstrate, a Committee chooses a job that seems best to suit their  inclinations.  Although every effort is made to make "Assignments" correctly, they are not set in stone once given.  At the Ceremonies, the Chief Elder explains,

"Sometimes...we are not entirely certain about the Assignments, even after the most painstaking observations...so we continue to observe during training, and to modify...when necessary".

"Selection" is a whole different thing.  There is only one Receiver of Memory in the community, and when it is time for him to be replaced, his successor is "Selected", not assigned.  A "Selection", in the words of the Chief Elder, "is very, very rare".  The person who is "Selected" must have uniquely specific but almost intangible qualities which appear in the population very infrequently; this person must have the capacity "to see beyond".  It would not be too far-fetched to say that in seeking to identify the next Receiver-in-Training, the Committee is looking for a progidy.  Once the Receiver-in-Training is "Selected", there is no chance to rectify the decision; his behavior cannot be observed or re-evaluated, and there is no going back.  If the Committee is incorrect in its "Selection", the repercussions are grave for the entire community (Chapter 7).