Where does the router get the stats from?

[#1]

Just wondering, because it always seems to display the *maximum throughput* regardless.

[#2]

Depends on the stats you refer too.
If you man things like Noise Margin and Loop Loss, then these are obtained from the modem as a result of line analysis and data supplied to the modem by the DSLAM.
Other figures may simply be local information relevant to internal interfaces which may never change (a LAN cable reporteding 100Mbps at all times for example).
Any specific values you are referring too?