I think that's definitely a factor. I came into a project once where that had happened. The initial estimate was a mistake and the contractor decided to up the

I think that's definitely a factor. I came into a project once where that had happened. The initial estimate was a mistake and the contractor decided to up the estimate incrementally whenever they reviewed status. The client became annoyed because "every time they estimate completion, it is taking longer." (I had worked for both and came in as a third party and due to that, was able to get things back on track.) The initial situation brings me to the other major factor I see in these situations. Anything that was overlooked or comes up unexpectedly results in more work. The situations where unexpected situations result in less work are as rare as hen's teeth. This results in underestimating more often than not.