Hi,

The 600Mbps speed is the max speed for the powerline network e.g. the speed across your home electrical cabling between each powerline adapter. The speed you get will be dependant on your home electrical circuits. On my powerline adapters I get anything from 100Mbps to 400Mbps.

The 100Mbps speed is the max speed that is supported on the powerline network ports. Therefore if your home internet is 750Mbps, you will only get 100Mbps using ethernet cables. If you want higher speeds then you need to purchase powerline adapters with 1Gbps network ports here - https://www.tp-link.com/uk/home-networking/powerline/tl-pa717-kit/

Hope that helps and makes sense.