Purely luck in how your house is built is most of the answer.
Those powerline units are likely older models. Tplink for some reason renamed them av600 even though I think they are the same as the older av500 models. I think they did this to cause confusion with the newer AV2-600 devices.
First you will never get more than 100mbps on those powerline units. They only have 100mbps ethernet ports. Sorta of a admission the 500/600 number is a massive lie.
They make newer devices that will run faster. AV2 models tend to run better so a av2-600 will be a bit faster. AV2-1200 is pretty much the fastest you will find. The 1000 and 2000 models from what I can tell are not different technology just different names. Even...