Hi Voyager888.
18-21 klms/ltr is roughly equiv to 51-59 mpg. Its not really possible on a CT with 2 up on a motorway at 120klm, its only a "spot check" of your current situation, may'be you roll off the throttle behind traffic or are on a slight decline, I have watched this metering also on the CT, and when it starts up an incline it takes 5 seconds to adjust downwards and then upwards when back on the flat. You need to take an average of about 15 - 16 klms per 1 litre and try to adjust your riding style around this.
On bigger trips like yours I find the consumption does not increase at higher motorway speeds ( like 140kph) on average compared to riding to work etc.
Cheers