Mind the last 20%!!!

If there is an urban legend that has grown in our agency over the last 10 years building ever more ambitious Drupal sites, it is to warn new project managers to fear the last 20%... However this does not prevent them from falling again into the trap. While the project was well under control, they see it exceed the initial budget or timing. What makes it happen so often? Is this specific to Drupal or does it also happen with other CMS or even common to any software development project (90-90 rule) ?
If this session is selected we intend to spend more time analyzing the reasons behind this unexpected overrun, survey other agencies to verify if they have the same experiences and share tips and tricks to avoid it and keep global control.