There are differing ways of measuring seat width and 17.8" is probably correct. UA's are of similar width and they just rounded it to 18." Most Airbus narrowbody coach seats are about 17.8" wide and most Boeing narrowbody seats are 17.2" wide. Some round those numbers to Airbus 18" and Boeing 17" but regardless, the Airbus is noticeably wider. VX is probably measuring from the outside of the armrest or something.