The date range selector enables users to change the selected period when viewing the widget on a dashboard, among a set of values.
On the widget record form, on the Date Settings tab, the Period field dropdown allows you to select among a different set of values.
As such, it is not possible to specify that a 6-month date range should be used when the widget is being loaded.
The Period values in the dropdown of the Date Settings tab on the widget form come from the Choice table records that correspond to the pa_widget.period field. However, the period values displayed on the date range selector do not.
This is working as designed. If the desired date range is not available in the Period dropdown of the widget form, you can instead use the date range selector on the rendered widget.