The unique thing about estimates in software engineering is that if you do it right, projects should be impossible to estimate!
Tasks that are easiest to estimate are tasks that are predictable, and repetitive. If I ask you how long it'll take to add a new database field, and you've added a new database field 100s of times in the past and each time they take 1 day, your estimate for it is going to be very spot-on.
But in the software world, predictable and repetitive tasks are also the kinds of tasks that are most easily automated, which means the time it takes to perform those tasks should asymptotically approach 0.
But if the predictable tasks take 0 time, how long a project takes will be dominated by the novel, unpredictable parts.
That's why software estimates are very hard to do.
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