This is because, although an additional amount of money corresponding to the traveling distance is added to fares on the regular railway line to the west of Shimonoseki, operated by JR Kyushu, such an amount of money is not added on the Shinkansen line operated by JR West.
JR九州管内となる下関以西の在来線では乗車距離に応じた加算額が課されるのに対し、JR西日本管轄の新幹線ではそれがないため。