Nope, nothing's changing other than switching from one cable that's going via the homeplug by the PC, for another cable that's running down the stairs directly to the router. Although the second set of tests seems to indicate that it's not the homeplug effecting the route, just the pings.
IIRC they do have some sort of inbuilt QoS system, perhaps that's effecting the ping. They certainly seem to throttle speeds, because while the connection between the upstairs homeplug and the one by the router is 200Mbps+ the throughput speed hovers around the 50Mbps region.