According to the FCC doc [1], the measurements may be conducted "between two defined
points on a network, such as between a user’s interface device and the ISP’s network core or between the user
interface device and the nearest internet exchange point where the ISP exchanges traffic with other networks". They require these measurements to be an average. I think this makes sense - it at least guarantees your speeds within the service provider's network.
[1] https://docs.fcc.gov/public/attachments/FCC-22-86A1.pdf