Maybe too much electric noise has effected the speed test performance.

Anyway you don't need to pay much attention on this kind of test result as it is not accurate. The important thing is you can use your product normally.

Are you getting 50Mbps when connected directly to your main router? If not, then try rebooting everything

I bought the TL-PA4010KIT few days back and I can only get between 40 - 50mbps from my 100mbps fibre-broadband package.
I am able to get about 110mbps connecting directly to the modem/router. Wirelessly, I am able to achieve 75mbps in my room.
When I connected the pair of 4010 on a power strip side by side, I was shocked to see the speed of 55mbps.
Now I can only conclude that there is nothing wrong with my 100mbps package and nothing wrong with my home power line.

the max you will get is 50mb. doesnt matter if on 100, 200 or like me 300mb internet the max this homeplug can give you is 50mb