makes no difference...
at the I-485 stage, there is no preference based on category, and EB-1, EB-2 and EB-3 are all treated the same. as far as i know, the only cases which get expedited are the so-called "age out" cases, where a derivative applicant is about to reach the age of 21.
so unless DPIL has a 20+ year-old offspring, it could simply be NSC has reached the load factor in Nov cases, and has starting touching Dec cases. because of the recent AVM/online problems, the data for Nov is not accurately reflected on rupnet.
or it could be just a freak case. one swallow does not a spring make... we will have to wait for more Dec RD approvals or RFEs before reaching any conclusions.