When requesting data from the Data Warehouse with weekly granularity applied, week numbers are returned instead of actual dates. To complicate things, if a year ends in the middle of a week, that week is split into 2 separate week numbers, making it difficult to nearly impossible to tie these week numbers back to actual dates. The requests end up returning more week numbers than actual weeks in the time period requested.
The simple solution would be for Data Warehouse to treat weekly granularity requests the way Report Builder does by returning actual week starting dates instead of week numbers. It would be even better if you could choose the date format you want returned the same way you can in Report Builder (week start date, week end date, etc).