As others have pointed out, T+2 is the standard settlement arrangement for most transactions executed on HKEx. There is nothing random about it.
Possibly what is causing confusion is that the broker will credit the purchased securities to your account with them immediately after purchase (so you could deal with them immediately if you wished) but this is only a book entry and is entirely different from what happens when the trade is settled though HKSCC/CCASS and Henderson's share register.
"Stock and money positions of all Exchange Trades and Clearing Agency Transactions are required to be settled on T+2 day whereas stock positions of China Connect Securities Trades are settled on T day. SI transactions are settled on the settlement day stipulated by both Participants."
Full details here: https://www.hkex.com.hk/Services/Settlement-and-Depository/Settlement?sc_lang=en