It looks like MANY cases fall into the primary applicant's 485 gets approved, and dependents' are pending. It appears to be caught by the 2% limit(from OCT visa bulletin):
"2. Section 201 of the Immigration and Nationality Act (INA) sets an annual minimum family-sponsored preference limit of 226,000. The worldwide level for annual employment-based preference immigrants is at least 140,000. Section 202 prescribes that the per-country limit for preference immigrants is set at 7% of the total annual family-sponsored and employment-based preference limits, i.e., 25,620. The dependent area limit is set at 2%, or 7,320."
GL