I wonder if there is still a source of interference within your house that is causing problems with the upper part of the spectrum. If the potential Upstream is 20 and you are consistently getting just 12 then it could be possible.
Have you tried turning OFF and unplugging everything else in the house apart from the Modem/Hub and PC? Then running another test. If that returns 12Mbps it would suggest external interference or noise but if you get 18Mbps then you will need to find out what causes the drop.
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