Interviews for ROC can get waived if the applicant already had an interview inside the US and my guess is that the same applies to biometrics. Since NBC is handling everything that has to be sent to a Local Field Office it does make sense that cases that don't fulfill these criteria do get sent there. Same for people that apply for naturalization while ROC is still pending. Their ROC cases often get transferred to NBC too to get combined with their N-400 and then sent together to a field office for interview.
Interviews for ROC can get waived if the applicant already had an interview inside the US and my guess is that the same applies to biometrics. Since NBC is handling everything that has to be sent to a Local Field Office it does make sense that cases that don't fulfill these criteria do get sent there. Same for people that apply for naturalization while ROC is still pending. Their ROC cases often get transferred to NBC too to get combined with their N-400 and then sent together to a field office for interview.