On the overview page for a survey, it appears that the "Last Submitted On" query does not account for resubmitted sent forms. I am guessing that it is looking at the "Created_Date" field. A better approach may be to look at the "Edit_Date" field, which will give a more accurate indication.
My field technician submitted a form on 8/7. To correct errors, he resubmitted the form on 8/12. The widget on my overview page says that the last submitted form was on 8/7, which is not completely true.