http://www.uscis.gov/files/nativedoc...8rev102108.pdf
16. Question: Although FIFO is the proclaimed order of processing at USCIS, there has been many observations on I-485 adjudication where people with later receipt date and priority date are approved ahead of others from the same country with earlier receipt date and priority date, especially at TSC. The delayed processing of I-485 cases with earlier priority dates is not only unfair to those petitioners, but also causes significant future visa bulletin retrogression given the large number of pending cases with earlier priority dates having not been adjudicated. What procedure or guideline by USCIS is in place to ensure FIFO in I-485 adjudication, and what additional measure USCIS can take to address the issue?
Response: FIFO is the order of processing at USCIS whenever possible. Cases filed with the same priority date may be adjudicated on different dates due to a myriad of reasons including incomplete documentation in the file or security check issues. The Nebraska and Texas Service Center work diligently to ensure that all cases that can be adjudicated are adjudicated. FIFO is the standard of processing. However, visa availability, security checks clearances, and evidentiary concerns can cause cases to progress at different speeds.