There are 2 different things your talking about. The 6 lbs your talking about is the weight of the air, from full tank to empty.
The advantage comes in to buoyancy. At the end of a dive the Al80 will want to float up at a rate of 4lbs; the steel 80 will want to sink at a rate of 3lbs. The difference allows you to drop not only the 4lbs to correct for the lift of th Al80 but an additional 3lbs which you will not have to compensate for. Which in theory allows you to drop @ 7lbs of lead.
The other advantage to a steel 80 is that the empty tank will wiegh less as well. An empty steel 100 weighs about what a Al80 does and is about the same size. Which will give you greater capacity without any additional total weight needed.
The reason the Al80s weigh so much empty is that for a given strength Al must be thicker.
hope this helps.
The reason that you get these weight advantages