Activity times may be estimated and are independent of each other. The variance in the length of the project is assumed to be equal to the sum of the variances of activities on the critical path. I have many critical issues and players that influence my project on a daily basis. Any statistical feeling of whether I'm ahead or behind of schedule doesn't guide my decisions. In theory, this means that if I have double the number of people on an activity, the activity will be done in half the time.
Software development is a process filled with uncertainties.
The progress of a software development project may rely on information being discovered or created in the process of performing the project. With projects with high uncertainties, the ideas of estimating, planning, sequencing, and progress seem out of sync with reality. These expectations can carry over into cost and specifications. The project team will spend useless effort on traditional overhead activities, like reporting, documentation, audits, evaluations, and administrative reviews, with no value added. Software project managers will face big dilemmas with customers and sponsors who don't recognize or aren't familiar with the difficulties of projects that involve high levels of uncertainty, exploration, invention, or discovery.