Hong Kong makes this more obvious by letting banks issue paper currency: HSBC, Standard Chartered and Bank of China all issue their own banknotes, with radically different designs to boot.
Northern Ireland and Scotland do this too, but there it's kind of obfuscated by both being interchangeable with the British pound, while the HKD stands alone.
That's the history as well, individual banks used to issue paper notes which were redeemable for something "real" (gold in the vault). This evolved to the system we have today where they did away with the gold.
Northern Ireland and Scotland do this too, but there it's kind of obfuscated by both being interchangeable with the British pound, while the HKD stands alone.