This would mean that there are hits where the Referring Domain is "checkout.telstra.com.au" and those are the first hits of the visits. I assume such hits don't fall under your "Internal" Marketing Channel's processing rules, nor anywhere else.
In that case, AA doesn't know what to do with them, and categorises them under the "None" channel.
It might be worth reviewing all of your Marketing Channel Processing Rules in a holistic fashion instead of troubleshooting one-by-one.