When we had our fibre installed the Transfield tech said you can't actually get the absolute maximum on your plan because system overheads consume a small amount of speed. So on our 200 plan we were told to expect a maximum of for example 199.
This is my paraphrasing of the tech's more detailed comments which I don't recall, so I'm sure any errors are mine.
Chorus takes into account of the overheads you mentioned when setting the speeds.
The "200/200" plan is actually overprovisioned as 217/230. Therefore it's entirely possible to get 200/200 on a speedtest.
In the first 2 years of UFB rollout people complained from seeing 93/45Mb/s on speed tests. The issue was diagnosed as the TCP overhead.
This is how the technology functions. There is no fault there. But people complaining had Chorus lift their profiles to the above speeds so people "Saw what they were paying for"