The 100Mbs is more than likely the speed that the Power Line Adaptors are set to.
If not the Power Line Adaptors it could be the device Network Interface Card [NIC] speed.
Older devices had NICs set at 10/100, they then moved to 10/100/100 then to 100/1000 and some are now set at 1000/10000.
If you go into the device settings you will see the NIC settings.
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BT Smart Hub 2 with 70Mbs Download,18Mbs Upload, 9.17ms Latency & 0.35ms Jitter.