The method Project uses to calculate Task Cost results in an inaccurate Task Cost value in the Project if, during the same update, you transfer both:
 
• One or more new resource assignments, AND
• Task Estimated Cost to Task Cost
 
To prevent this inaccuracy, transfer new resources and cost mapping for a task in separate updates.
If you have already transferred cost mapping and new resources for a task in a single update, retransfer the project from Web TimeSheet to Microsoft Project using the same mapping to correct the data inaccuracy.