http://www.tp-link.com/en/faq-461.html

Check this article.

500Mbps is the PHY rate of power line communication.
100Mbps is the Ethernet rate.

Reacher thank you!
That is exactly the answer I was looking for.
I can just keep on wondering why would the end user care about it being 500Mbps on the physical layer. I would care about the maximum achievable data rate possible of the product as a end-to-end.
I mean, as an end user, I don't really care about how the product internals work, just what I can get from it. It seems a bit misleading to me..
Just my opinion.
Anyway, thank you very much for the helpful information :)