I think you're right, I find this on the NVC contact page for immigrant visa :
Interviews
The U.S. Embassy or Consulate tells NVC which dates they are holding interviews. NVC fills these appointments on a first-in, first-out basis and cannot predict when an interview will be scheduled.
Once complete, your case will remain at NVC until an appointment is scheduled, at which time we will send it to the appropriate U.S. Embassy or Consulate. We will notify the applicant, petitioner, and attorney (if applicable) when an appointment is scheduled.