A careful read of the USCIS website would indicate...
"The processing time displayed on the USCIS website is the amount of time it took us to complete 80% of adjudicated cases over the last six months. Processing time is defined as the number of days (or months) that have elapsed between the date USCIS received an application, petition, or request and the date USCIS completed the application, petition, or request (that is, approved or denied it) in a given six-month period."
and before you ask the next question, these times are updated monthly.