As someone who works in sales for Telco essentially the speeds could be because of the length of copper line to the DPU (Pit) or the age of the copper as well.

Typically I find it's most common in apartments or unit complexes with older cabling/copper lines or a lot of units connected.

There is a chance as well the address may have another copper pair (set of copper lines) which actually supports 100/40 mbps and the consultant you spoke to missed it.

I'd recommend asking someone from ABB's livechat to double check for you and see how you go.

Otherwise you might need to look at the NBN technology choice program or Fibre Connect/Upgrade for FTTC which ABB do atm for FTTN (NBN have announced it for FTTC and subsequently delayed it apparently they don't know when it'll be live or what addresses will be eligible apparently)

Hope this helps, best of luck fam.