Collecting all my different addresses in a spreadsheet, I realized that on the exchanges "Huobi" and "Gate.io" my BTC Wallet Adresses were identical to my USDT Wallet Adresses on these exchanges.
USDT is a token handled by the OMNI network* which rides on top of Bitcoin. OMNI transactions are Bitcoin transactions with a few extra payload bytes. Thus, USDT and OMNI addresses are simply Bitcoin addresses.
It would appear that Huobi and Gate.io are simplifying the management of keys by using the same addresses when possible.
* USDT also now exists on the Ethereum network, but this is a new variant.