Project Online Content Pack for Power BI - Portfolio Costs Report - Sum/Detailed Differences

I'm seeing differences between cost data (EAC(Cost), Actual Cost, Remaining Cost, and Cost Variance) in the 'top half' when compared to the 'bottom half' of the 'Portfolio Costs' report in the Projects Web App Dashboard for Power BI.

Fields in the 'top half' are drawn from the 'Projects' table. Fields in the 'bottom half' are drawn from the 'Assignments' table. From my perspective, the cost information in the 'top half' is correct - ie they reflect the current values in the Project Professional built-in 'cost table' view.

In the displayed project, I assigned a cost only directly to a single task - as a means to have baseline, EAC, actual and remaining costs appear on a dashboard for the Executives. The task is 100% complete. There are no resources assigned to any tasks.

What is the purpose of the 'Assignments' table? Are 'Projects' table fields copied to the 'Assignments' table, and if so, when or what user action triggers this?

Moreover, how should I be applying / updating costs so that there is no anamoly between the 'top half' and 'bottom half' of the report?