Mustang EcoBoost - suspiciously high fuel consumption below 12mpg

I have noticed that my EcoBoost Mustang has an alarmingly high fuel consumption when cruising at higher speeds on the freeway/motorway. I usually cruise at 160km/h (100mph) with 12 l/100km (19.6 mpg) which is acceptable, but once I go 200 km/h (124 mph) or faster, the fuel economy drops significantly to 18-20 l/100km (13-12 mpg). Yesterday I recorded my worst result yet with over 21 l/100km (below 11.2 mpg) which is a very poor figure by any standards. My EcoBoost Mustang has the worst average fuel consumption from all EcoBoost Mustangs on Spritmonitor - am I doing something wrong or is this normal and I should just get used to it?