There is always space consumed by "overhead"; space needed for directories, organizational stuff and "medium bookkeeping".

I don't believe I have ever had a disc or disk that made the specified capacity completely available for use.

On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.