For example, a direvative and an unrelated primary both have a PD on 12/01/04. The pending primary might wait for 3-4 years before 485 can be approved. But for the direvative, as long as his/her primary has been approved, he/she might only need to wait for a couple of months.
Note that the employment based visa number is still available but limited. If the visa number is completely out, the visa bulletin will say "Unavailable" instead of retrogression back to year 2000.