Authorized shares are a ceiling, not a count of anything trading. The company issues some of them, holds some back for employee plans, and can sell more up to the ceiling without asking permission again. That headroom is the fuel for future dilution.
Reading the gap between authorized and issued-shares tells you how much a company could issue tomorrow through an at-the-market-offering. A proposal to raise the authorized count in the proxy-statement is often the tell that an offering is coming.
Example: a company has 500M authorized and 120M issued. It can sell up to 380M more shares without another vote. At $4 a share that is $1.5B of potential dilution sitting in the charter.
Related: issued-shares, outstanding-shares