The week number is calculated, across months. Week 27 may be in June and July. If your found set is July, but you create a report to subtotal on the week numbers. the first week and last week of the month will not be 7 days. If you select whole weeks, you will get five weeks, 35 days.
You would find the month. Then do a find that is by the first and last week numbers in the month. That will extend the data.
"I want to track this data on a Sun - Sat basis"
Go to play with the WeekOfYearFiscal ( ) native FileMaker function.
WeekofYear() works on the default of Sun-Sat.
If you wanted a different workweek you would use WeekofYearFiscal() you can option to set a different start day.