(Figured I’d move it to the right thread)
This is because of HFC which is currently capped at a sustained 750mbps with bursts up to 1gbps unlike FTTP which can sustain the full 1gbps.
They use 600 as a baseline safe speed.
Telstra has fully vacated the Cable network now so we should begin to see improvements to HFC over the next few years.
NBN plan on enabling the 250/100 tier on HFC by May 2024 so improvements are already on the way.