500Mbps is the bit rate on the medium (power wiring). Due to noise and interference, the bits sent include a huge number of overhead bits for "forward error correction". Another speed reduction comes from the medium being used in half-duplex (cannot send/receive simultaneously, just like WiFi). Fast receive helps reduce overhead as receive is used for ACK frames and so on.
This, and other factors, cause the net yield to be a fraction of the medium bit rate. Like WiFi's net yield about 60% of the indicated raw bit rate.
Still, it seems that if there's a good strong signal, it ought to get more than 100Mbps net. But 100Mbps net yield is very, very high for other than wired ethernet!
Consumers need to read between the lines.. just like in cars, a 200HP engine cannot deliver 200HP to the tires.
