In my experience software takes longer to build than original estimates because no one will get out of the way of the development team and let them work.
This is an extreme example, but one I now live in daily.
My current full-time-ish gig is working on a pretty enterprisy system for law enforcement. To this date there hasn't been a single feature request, or bug fix that took more than 16 hours of development time. And so I know that I can typically finish something within a few hours to a day of receiving the task. UNLESS my manager wants to discuss ad nauseam what he means when he says "intersect an array". Or get stuck in 2 day long code reviews where my manager makes me sit behind him while he goes over every single line of code that changed, then gets side tracked and starts checking emails, chat messages, text messages, calling other developers in to check on their statuses, and even watching youtube... while I'm stuck in his office waiting on my code review to be done so I can go back to my 5th day of trying to complete a task that would have taken only a couple of uninterrupted hours. /rant
I give estimates in hours. They're really by days for large projects and half days for smaller projects, but I express them in hours. I then provide a breakdown of the hours the team has been able to spend to date, as well as the hours that each forced interruption took. That way, when my manager asks why he perceives us as being behind on the project, I can tell him exactly how long his pointless meetings and changing priorities have delayed the team. It's also a good CYA technique for when he tries to blame the team when his manager asks why the project is delayed.
I find it useful to point out in situations like this that we have already spent more time discussing the issue, or often "how not do to the thing" than it would take to simply do it.
He was originally hired as the sole developer 10 years ago, but the project grew too big, and instead of hiring a manager to oversee the project and hire more devs, they moved him into a management position, and put him in charge of hiring new developers.
This is an extreme example, but one I now live in daily.
My current full-time-ish gig is working on a pretty enterprisy system for law enforcement. To this date there hasn't been a single feature request, or bug fix that took more than 16 hours of development time. And so I know that I can typically finish something within a few hours to a day of receiving the task. UNLESS my manager wants to discuss ad nauseam what he means when he says "intersect an array". Or get stuck in 2 day long code reviews where my manager makes me sit behind him while he goes over every single line of code that changed, then gets side tracked and starts checking emails, chat messages, text messages, calling other developers in to check on their statuses, and even watching youtube... while I'm stuck in his office waiting on my code review to be done so I can go back to my 5th day of trying to complete a task that would have taken only a couple of uninterrupted hours. /rant
And this is why I pay $120 a week for therapy.