It seems that you only count as "backlog" if you are a case that they can process without any problem whatsoever.
In my case, they "descheduled" my interview because of a USCIS screwup (they District Office "ordered my file" which did not show up in time). Once I was off the "fast track", they appeared to have no interested in me. Interestingly, while waiting for my interview today, the lady beside me had had her early May interview descheduled. While I was able to get mine back on track by leveraging the staff in my senator's office, she paid for a lawyer.
Grrrr.