Toi, it’s originally the Chidushei Harim’s kasha (though R’ Chaim may have added reasoning in saying it has to have been “kasis”). And both the Chidushei Harim and R’ Chaim are muchrach to answer that there is a difference between oil that “expanded” in the proximity of “real” olive oil, whatever that means, and olive oil created from nothing. However, in light of the answer I provided above that the oil never decreased until the eigth day, we do not need to be forced into that explanation for this question.
Shticky Guy’s answer about making sure Beis Hillel and Shamai’s opinions never align so therefore no seven days also answers the question of why not nine days because of sfeika dyoma. -Gra