Virginia,
To better understand this, I would need to gain access to your environment and specifically look into the Projects, Tasks and Time entries as well as this reference in the Custom field called $R_UnbillableTasks.Duration and exactly what that is referring to (i.e. Total Planned Work or Actual Effort). If it is looking at Work instead of Actual Effort, this info will be skewed.
Feel free to open a ticket, attach the docx and provide the Grant Access URL for access to your environment.
Sincerely,
Boris