I don't think there is any attempt to reduce width on narrow body planes, on them seat width is basically determined by cabin width. 17 inches means you are on

I don't think there is any attempt to reduce width on narrow body planes, on them seat width is basically determined by cabin width. 17 inches means you are on a Boeing 737 derivant, 18 inches means you are on a Airbus A318/319/320 derivant. The decline in average width probably means a higher percentage of 737 flights. That increase could be driven by retirement of older planes such as MD-80s that had an 18 inch seat width.