To me the main difference is temperature. When adding hops at flameout the wort is still close to boiling temp so the volatile compounds (oils) are being driven off until the wort has cooled past the volatization temperature. Aroma steep I will usually cool the wort to 180 then add hops and let them steep for 20-30 mins. Since the wort is cooler less oils are driven off and there for are still in the wort to give you aroma which translates in to more perceived hop flavor. You can even do multi step aroma steeps since there are different volatization temps for the oils found in hops.