Check out the difference between effort driven tasks and non-effort driven tasks.

2. MS Project Tips and Tricks – Effort driven or
Non Effort driven?

3. The Task Effort is the sum of the Work (hours) of all
Work Type resources assigned to a task.
The Effort driven option determines how the effort is
calculated as resources are added or removed from a
Fixed Units or Fixed Duration task.

4. There are two options in MS Project:

5. Effort Driven - When a resource is added or removed
from a task, the Task Effort assigned to a task remains
constant. Therefore the Work of existing Resources is
reduced when a new resource is assigned.
Adding or removing resources from a task will leave the
total effort assigned to a task constant, unless all
resources are removed or a change is made to the work
of existing resource assignments.

6. Non Effort Driven - When a resource is added to or
removed from a task, the Resource Effort or Work of
other resources remains constant. Adding or deleting
resources increases or decreases the total task effort
and will not change the effort of assigned recourses.

7. In summary, as you assign resources:
-If you want the total number of hours assigned to stay
constant then make the Task Effort driven.
-If you wish to assign each resource with its own hours
or units per time period then make the task Non Effort
driven.
Note: A Fixed Work task is automatically Effort driven.

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