Project managers believe they are managing a project, but a project is a concept and you don’t manage concepts. You manage people (and other resources) in the delivery of activities to meet a specific objective/requirement set by other people.
People change their minds. People make mistakes. People get sick. People disagree. Infrastructure fails to perform and budgets shrink …
Successful project managers recognize the dynamic nature of projects and that project plans are nothing more than a reflection of our current understanding of the situation and a prediction of our ability to meet the known objectives/requirements. The plans must change to adapt to the changing situations.
Ultimately, success depends on how we recognize and adjust to the never-ending changes which makes situational awareness a critical (and yet over-looked) skill. That does not mean that a project manager shouldn’t do everything in their power to minimize variability and increase predictability but they must accept the dynamic nature of people and their impact on projects and manage accordingly.