Triumph should definitely sort it (though if it's out of warranty now maybe not)
Like your Rocket, my Harley under-read by about 10%, but I'd got so used to speedos over-reading by about 10% I always assumed that the Harley was doing the same thing.
Once I checked the speedo with a GPS and discovered that the speedo was out in the wrong direction I realised the bike was nowhere near as slow as I thought it was. When the speed showed 70 I thought I was doing 63 but was actually doing 77
Of course I always thought that a true 80 would be ok on the motorway, so would cruise at up to 88 indicated, which turned out to be about 97. No wonder the thing was running out of puff.