Report Group by Date Aggregation

I have a report that is setup to do grouping by date and aggregate to week. Shouldn't the date that it is showing in the grouping be either the first day or last day of their 'week' from their usage configuration?

You can see here the report is showing Monday as the day when I am expecting either Sunday(beginning of week from their usage configuration) or Saturday(last day in the week).

I could not find the logic being this in the support documentation for this feature:
https://support.gainsight.com/Product_Documentation/Reports_and_Dashboards/Admin_Configuration/Repor...

Thanks!