It’s anybody’s guess | The Register

"However, unlike with my train journey, it is very unusual for software developers to be asked to estimate from a position of certainty. To gain a competitive advantage, organisations need to be delivering new and, hopefully, unique functionality. 'New and unique' is, by definition, something we have not done before and have no experience of. It is, therefore, unknown."

Good coverage of why things go wrong, but there is more.

Of course it doesn't cover government contracting. There are additional factors here. For example, your government "management" was supposed to tell you to start working on a new feature in January, but they either forgot, or failed to attend the internal meeting where this information was passed down. So, when you actually begin work on the "planning" phase of the project in July, you post date all the items on your project plan to make it look like you actually started work in January. While you're still trying to figure your ass from a hole in the ground the schedule says you are midway into the coding phase. You still have to go to status meetings with your government "manager's" peers and engage in fantasy-land talk so as not to embarrass the bozo/bimbo, and it is you, not the bozo/bimbo who must take that lashes for things being "not quite finished yet". Of course, reporting the facts of the matter is out of the question, as your "manager" and the people you would report him/her to are all in this lifetime employment scam together and are probably at least as incompetent as yours is.

Every now and then the cumulative disparity between your fantasy-land project and reality actually has an impact on something that the public might become aware of. But the good news, is that by then, there is so much blame to go around that it is indistinguishable from nobody being to blame. Steady as she goes, slower, slower slower: The game goes on.