Hong Kong Currency Notes

Early this month I travelled to Hong Kong while returning from the USA. When I converted my money to Hong Kong Dollars to spend locally, I noticed they had a difference. Unlike currency notes of other countries say India, US or UK which are issued by Central Banks of those countries, the Hong Kong currency notes seem to be issued by authorized private banks like Standard Chartered, Bank of China, HSBC and others. All these are legal tenders in Hong Kong.