Congrats!!
Actually, I later discussed this with other people; someone pointed to me that DOS allocates a visa number when they schedule an appointment.
"Visa numbers are always allotted for all documentarily qualified applicants with a priority date before the relevant cut-off date, as long as the case had been reported to VO in time to be included in the monthly calculation of visa availability."
(Last paragraph in page 2 of this document: https://travel.state.gov/content/dam/visas/Immigrant Visa Control System_operation of.pdf)
So I guess people who are included in the "monthly calculation of visa availability" actually already have a visa number allocated. And since the monthly calculation happens at the beginning of a month. (On the visa bulletin of September, one can find that the date was August 2nd). In the rare situation where the visa is eventually refused, this visa number will be returned to the pool. Otherwise, I think people with interviews scheduled in September should have their visas issued as usual.