Don't stress. Are you really really sure your BCD was empty? Also, if you are diving with a Jacket style BCD, you may actually need that much weight or more. They tend to make you more buoyant based on the way they change your shape (and thus the buoyant force acting on you). AL80 tanks tend to get more buoyant as you use the gas in them, so there's also that. Consider asking the DM to do a weight check with you on your next dive.
This is a misleading statement that never seems to go away. The tank material has absolutely no bearing on the CHANGE in buoyancy during a dive, which is what we use a BC for. All tanks get more buoyant as gas is used from them. 80 cf of gas used will cause the exact same change in buoyancy whether it comes from an aluminum tank, a steel tank or one made from any other material. How much weight needed to compensate for the tank material does vary depending on the tank material but once that is determined, it remains a constant throughout the dive.