It's not just that, but also we tend to estimate in the context of "if I were sitting at a computer working on just this problem, this is how long it would take." The reality is that there are meetings, high priority bugs for unrelated systems, interruptions from the business, coworkers and life, code reviews for other team members, rediscovering what you were doing before being interrupted, etc.

Using time tracking, I was able to discover I only spend 2-3 hours per workday actually programming, the rest was all interruptions and such. Thus I can estimate that one day really equals 3-4 workdays. Then my project manager throws in another 3-4x on top of that to deal with scope creep, rework, bug fixes, etc... and we're usually on-target ~50% of the time.