In Default order settings, tab page Details, section Sales order, users can set check box OVERRIDE SALES LEAD TIME to change the value in Sales lead time field. This value overrides the default Sales lead time set in parameters.
When you however close Default order settings form, and open it again, you will see that the Sales lead time field is again disabled, and user cannot edit the value. Only when the user switches OVERRIDE SALES LEAD TIME field to No, and back to Yes again, the field becomes editable.