Faulty Construction...

I've been reading, and with no small interest, many of the recommended materials on constructivism. I've read some things more than once because each time I do, I have the same sort of peculiar experience. I read and line by line I find nothing objectionable. I even find some things that make me want to stand up and cheer, "These are noble sentiments." But I get to the end and I think there's something wrong here; this is intellectual sloppiness.
As I understand it, constructivism is based on the recognition of the learners role in creating knowledge. It comes off like a repudiation of an imagined idea of how education used to be. Any well intentioned educator would surely agree that the old school model, where the teacher (the giver) stands before the class (the receivers) and through erudite lecture is able to fill those students with wisdom, does not apply to our world. One wonders if it ever did. Little matter, because we know better now. Our classrooms are learner centered. We allow students to cultivate their interests and thereby create their own knowledge. They made it. They own it. Great.
Here the thing; while we can all appuald the way in which a learner's unique experiences go towards building his or her inventory of knowledge, we're wrong in assuming that each learner "constructs" knowledge. There's a word for each learner's unique consciousness. It's called subjectivity. The process by which individuals come to knowledge is subjective. Knowledge itself is not.
I can offer easy proof. How many of us, when grading a test or quiz, have written 100 on top of every paper, thinking that if each learner constructs knowledge on their own, no one can be wrong? Who has encountered the student who, because of her/his unique experiences, determined that 2+2+5? Knowledge is objective.