well, the guy who received GC within 45 days of i485 application may come and say like u telling GIG that he should already have got the GC. That's why we go by average processing time.  

if you apply i485 once ur PD is available, it doesn't matter whther u r in eb2 or eb3 ,same processing time. 
are u sure it took 6 years after ur PD was availalable and after u applied i485?