Good blog post here how they try to make it an accurate, real world test.
On 2degrees fiber 100/20 plan in Wellington on ethernet, gigabit internal network, I get 103Mbps down with 15.7ms of latency and 1.48ms of jitter. They've disabled upload testing because apparently they were finding it inaccurate on very fast connections.
Interesting, my phone gets 102Mbps with 4.5ms of jitter. That's better than I'd expect. Fritz 7590.