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The speed tests find servers from many providers not specifically your ISP, so it could be TPG, iiNet, Telstra, Optus or many others. Usually it will choose the closest server to your location, in your case it would appear iiNet although no longer your ISP is the closest server for it to test from.
For more details you may visit this link, https://help.speedtest.net/hc/en-us/articles/360038679834-How-does-Speedtest-select-a-server-.
Let us know should you require further assistance.