Why is it that "90% done" (and its partner in crime - the ubiquitous "almost done!") is the progress report for virtually any project, over virtually all of its life-cycle?

Why has 90% become the fictional number of choice? Why not the more conservative 80%, or the bolder 95%? Given that it usually has little correlation with reality, they're just as real.

Projects should be reported as 87% done. Even when there's the ominous "we'll solve that problem when we get to it" task maliciously eyeballing you from later in the project plan, or the "it doesn't work and we have no clue why?" runtme reality, still say 87% with confidence and pride.