If you gained AND kept 12 lbs from that small amount of AS then you were NOT ready to be using AS in the first place
If one is WAY below their genetic potential (i.e. started taking gear when one was 140 lbs then YES they could gain AND keep that much), but an advanced athlete with YEARS of training (Which IMO are the ONLY ones who should be using steroids

) is NOT going to get those kind of gains from 5 weeks of 20 mg/dbol a day