It is averaged out! The September to September figure show the rise between those dates..... If the October to October figure is higher then that will mean that next year the September figure will reflect any real change that has happened in Sep or Oct and it will presumably be higher.
Some years the Sep figure may seem lower, but on others it will seem higher but over all it will average out. You have only noticed it because it seems lower than the surrounding months, on other years it will have been higher and you will not have given it the time of day