I'm starting to think that the culprits are the tests themselves. Instead of always using the same test website (http://www.speedtest.net/), I decided to try several other test websites and discovered that some are giving me ~7.5 Mbps and some others ~11 Mbps.
For instance, this one (https://testvitesse.videotron.ca/index-en.html) is always giving me 11 Mpbs. Odd.
Is there another way I can test the speed of the adapter than using [unreliable?] websites?