Previously you can file 485 only after 140 get approved.
So before you file 485, INS will apply for a visa number for your case. In this situation, whenever your 485 application has been accepted, you actually has a visa number.
When people are allowed to file 485 before 140 get approved, the situation changes. INS will accept your 485 application, but INS will apply for visa number until your 140 has been approved, your Name Check from FBI has been cleared, in summary, INS will apply for visa number until your case is ready for adjustment (apply visa number become the last step in GC application).
For those whose 485 is pending, INS will not apply visa number for them even if their cases are ready for adjustment. When their PD become current (PD is early than cutoff data), INS will pick up them and apply visa number for them, and the GC is approved.
I hope this explain a lot of person's concern.