Ever noticed how academic asses are analog and industrial asses are digital? It's legitimate to not know whether P equals NP, or to not know what x is if x*2=y but we don't know y, for that matter. But it isn't legitimate to not know how many cycles, megabytes or – the king of them all – man-months it will take, so numbers have to be pulled out of one's ass.
The interesting thing is that the ass adapts, that the numbers pulled out of this unconventional digital device aren't pure noise. Is it because digital asses know to synchronize? Your off-by-2-months estimation is fine as long as other estimations are off by 5. But it's not just that, there must be something else, a mystery waiting to be discovered. We need a theory of computational proctology.
Ever noticed how painful the act of anal estimation is for the untrained, um, mind, but then eventually people actually get addicted to it? Much like managers who learn that problems can be made to go away by means such as saying a firm "No", without the much harder process of understanding the problem, not to mention solving it? Anal prophecy is to the technical "expert" the same raw enjoyment that the triumph of power over knowledge is to the manager. "Your powers are nothing compared to mine!"
There once was a company called ArsDigita (I warmly recommend the founder's blog and have his Tenth Rule tattooed all over my psyche), a name I tend to misread as "ArseDigital" – a tribute to an important method of numerical analysis and estimation in the computing industry.