Summary:
If you add an assignment using Web TimeSheet's MSPI (MS Project Integration Manager) for MS Project server, the resulting Task Work field in Web TimeSheet will be different than if you added the assignment manually.

Cause:
This is because MSPI uses MS Project Server's application programming interface (API) tool, which uses the below given formula:
Task Work = Task Duration * 8 * Number of resources assigned to task
This results in Task Work being updated.

In contrast, manual entry uses the below given formula:
Task Duration = Task Work / (8 * Number of resources assigned to task)
This results in Task Duration being updated.