FP and NC relationship

For my case I485 RD 1/18/2005, NC send 2/2005. FP 7/1/2005, FP cleared 7/6/2005, and NC still pending.This result make me think FBI do these 2 different checks separately . ( Agreed by quite a lot of people.) But also some people suggest FBI do NC and FP together. The only possibility of that is that FBI hold your NC till they get you FP, then start processing both checks.But both logically and technically this is very difficult to do since NC is sedning to FBI by tape from CIS with thousands of applicants on that( refer to FBI web site), how FBI knew which to check and which not? But this is so critical question since it will direct realted to how long FBI dealy the name check. Does any one have knowledge from inside FBI?