The view of postmodernism correctly interpreted (I believe you call it 'descriptively') contends that “the current ‘truth’ is contingent — and may some day be called into question by any number of new factors or discoveries.”
This kernel of truth at what seems to be the root of postmodernist thought is present in other places that predated it.
In science the truth is contingent on our latest understanding, always open to re-evaluation. Like Asimov said, something to the effect of 'a scientist always reserves the right to change his opinions in light of new evidence'. Science has never been right. It's always been wrong, it's merely been a history of building on the least wrong (we hope).
http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm
In science it comes in riding on the consequence, empirically. We've never once yet been right. We are shown again and again that we are simply less wrong. All thought within science is always contingent upon a flawed and limited understanding of the external universe which is subject to change and open to multiple interpretative theories.
The same kernel concept is present in the philosophy of existentialism, that an individual stands alone from the world, where it's approached rationally from the inability of philosophers to escape their own perception. It is not, necessarily, that there is no objective world apart from the individual but that the individual cannot perceive anything outside of himself save but by perception, and the subjectivity of perception signifies either a fallibility in perception (of an objective existence), or a subjective existence. The former being quite correct, the latter being something of a solipsistic abuse that renders itself and everything else meaningless. It so much as 'deconstructs' itself.
Postmodernism is the same applied to linguistics. As science cannot describe the world outside our flawed and incomplete understanding, existentialism cannot describe the world outside our own flawed and incomplete perception, postmodernism cannot describe the world outside our own language.
In historical terms I can trace this handy bit of skepticism back to Ancient Greece with Solipsism - which also contained this truth as a 'philosophy of skepticism', and was also prone to the same misuse and abuse as an end run around to proclaiming ‘no one knows nuthin so don't bother to try’ relativism.
Even Socrates had it – “If I am the wisest man, it is because I know that I know nothing.”
Derrida's theories and practice of literary criticism and deconstruction applied it to literary interpretation. We cannot know meaning outside our own interpretation of it.
All are different approaches to the same point in space. Different schools of thought, different departments, articulating it in different ways or applying it to different places, but it seems to me the same bit of truth.
And what it is, is the statement that our perception is subject to human fallibility.
Now postmodernism gets entirely too much attention for that. It does not seem to me quite remarkable to have made such a fuss. Applied descriptively (to say, Palestinian narratives), it tells us only that 'people are wrong because they are fallible'. If people are wrong, then people must be fallible because they are wrong, which makes this a fun little bit of circular rhetorical tautology.
Descriptively, it is useful only in that it cautions us to be aware of the possibility of errors caused by the bias of perception. To properly take it into account is to merely make a statement of intellectual humility. 'As best I can currently tell'. Prescriptively, it prescribes for us to be open minded.
I am not sure why postmodernist thinkers get to attach the name 'postmodernism' to this, as though they were first to discover intellectual humility. (HAH!)
It seems to me certainly in the case of Derrida, and to my fallible and incomplete understanding, all of postmodernism, it has been abused from the get-go. Derrida outlined it precisely with the intention of misusing it. Which is the 'correct' use of deconstruction? BUT - Not which use of deconstruction is correct, but which usage is correctly CALLED deconstruction? It seems to me, especially with Derrida, that there is a case the name applies most accurately to his misuse of those principles he observed. Because what he did correctly, he did not do first. Even in just the literary world, aren’t these things often credited to having been done by the likes of Jorge Luis Borges before Derrida? What Derrida has invented, what remarkable new thing he has done that requires a name that we may refer to it, was his misuse of the technique to try and destroy the possibility of meaning. Based on the example of it’s inventor, to demagogically abuse the underlying principles involved is to properly use the technique of deconstruction. To properly use those same principles is something else entirely, to which Derrida neither deserves credit as a pioneer, nor has any apparent affiliation nor even history of practice.
Who then is to say what the correct "postmodernism" is? Not in the sense of which postmodernism is correct with regard to reality, but which theory is correctly represented by the NAME “postmodernism”?
It is something like me calling myself a 'liberal' based on an archaic definition that’s no longer in use. Regardless of whether I view the reason for the change in it’s meaning as valid, it has changed and it no longer denotes that in our language.
Most all of the fuss it has made, most all of the use to which it has been employed, has been in the improper way. To such an extent that I think that it is those who 'misinterpret' it who are correctly interpreting an incorrect theory, and those few meek literary and philosophical professors who nibble at a much smaller, less ground-shaking theory of 'postmodernism' are those who are incorrectly interpreting an incorrect theory in such a way as to be correct, and also largely unremarkable.
'Postmodernism' is most useful to describe the incorrect theory, which while incorrect, is at least remarkably so, with a substance worthy of a name. And that is how it has mostly been used. Why cling to the name to describe that (as I come to grips with it) 'Theory of Humility' in a manner I might try to cling to some archaic usage of "liberal" to describe my political beliefs? It seems to me any attempt to bolster the correct theory of postmodernism shall serve only to bolster the incorrect theory which has totally sublimated it's name.