Background
In versions of Microsoft Project prior to 2007, assigning a resource to a fixed-duration task did not affect the duration, even when resource calendar exceptions would otherwise cause the scheduling engine to reschedule the assignment. In order to achieve this behavior in earlier versions, the system created these assignments as zero-work assignments. This tricks the scheduling engine into leaving the duration alone, unfortunately, it distorts the system’s ability to provide accurate resource loading.
Resolution
Inasmuch as entering the EPM space has caused a shift toward a high demand for accurate resource loading projections, the product team made a decision to have fixed-duration tasks honor the general rule is that a task is a rollup of all its assignments. The product team removed the exception that existed for fixed-durations tasks in earlier versions when it released Project 2007. The new behavior better supports Project and Project Server as a work management system.
Because many users have expressed the desire to have fixed-duration tasks that ignore resource availability, we expect to see additional investments in this area for version next.