I'm not really familiar with the programming of the automated systems, but I think in many cases the number is forwarded and the person who answers the call does have your account on his screen. I think they ask for your account number anyway just to verify that everything worked and they're talking to the right person. They want to avoid the possibility of a privacy breach if some signals get crossed and they end up with a mismatch between caller and account.
That doesn't mean asking for your account number in advance is a total waste. When the live person asks you for it again, then all he has to do is verify that the one you give him matches the one on his screen, and if so, he doesn't have to enter it and wait for your account to appear.