We often find ourselves at a loss in explaining to business management why systems projects cannot be estimated as precisely as construction projects, why there are so many issues, why they cost so much, etc. I found the following excerpt from the book entitled Breakthrough Technology Project Management, by Bennet P. Lientz and Kathryn P. Rea to contain some good insights on this topic.

"Differences between Standard and Systems and Technology Projects

Purpose. The goals of a system and technology project are often not as clearly defined as those in engineering or other projects. The goals may not be well defined at the start of the systems project.

Scope. Systems projects sometimes lack clear boundaries. Is the business process within the project? With what systems must the project interface? Moreover, the scope can creep and expand.

Parallel work. While a new system is being created or installed, work can continue on the current system, creating changing requirements. This is not true in standard projects.

Interfacing projects. Systems projects are more likely to have complex interfaces.

Technology dependence. It seems that only in systems and technology projects do people attempt to use new technology or technology with which they have no or only limited experience, raising the level of risk.

Management expectations. Upper-level managers attend seminars and read about the promise of new technologies. Their expectations can impact the systems project.

Cumulative impact. One project can affect others. The latest project depends on the results of many previous and some current efforts. It is cumulative dependence.

Understanding the technology. Although non systems projects can involve technology, it is normally simpler to apply because the technology can be handled separately. In systems, it is the reverse. The only way modern systems can be successful is by integrating multiple technologies. This requires a deeper and more thorough understanding of the technology.

Technology gaps. Systems and Technology projects are also affected by gaps between the newest technology and the older technologies.

(Lientz, Bennet P. and Rea, Kathryn P. "Breakthrough Technology Project Management", Academic Press, San Diego, California 1999, pp. 5,6.)