Reply

The Final Action Date is set for the cases which have been filed before the Final Action Date and these cases can be approved, and the new I-485 cases which priority date  is earlier than this date also can be filed.

The Filing Date normally is set for the cases which will be processed by NVC for application for the immigrant visa outside US. NVC will ask the applicants to submit their file to NVC when their case priority date is earlier than the Filing date. Of course, sometimes USCIS also set the filing date for I-485 filing. In this case, you may be qualified to file I-485, but it does not mean your I-485 is qualified for approval until your priority date is earlier that the Final Action Date next time

So, the Final Action Date and the Filing Date in the same month are not related to each other. They have different meaning. So, it does not matter which is earlier or later.