Let's say for the sake of simplification that I have a project with 3 tasks: A B and C.
Task A is scheduled to be completed in 10 days. Tasks B & C are 1 day each.
Since all 3 tasks are being performed by the same resource they happen one after the other, making the project 12 days long.
Now let's say that we are on day 3 of the project and we find out that task B is completed (the developer had some spare time and was stuck with task A).
So I set task B to be 100% complete, and expect the project end date to reflect that change.
In real-life this would make task C start a day earlier and eventually the entire project would finish 1 day earlier.
This is not reflected in MS Project automatically, and I wonder if I'm missing a setting somewhere or some action to be done.
Best Answer
The short answer... because the MS Project team doesn't have to dog-food their own product.
Really MS Project isn't going to recalculate the rest of the project every time you change the completion of a value (and it may not be appropriate either). It can also screw things up nicely (ask anyone who has accidentally "rebalanced" their project - everything goes fubar)
You can sort of do what you want by re-leveling the resources:
Go to Tools
-> Level Resources
-> (select "Level only within available slack" and click OK)