Depends on the country I guess. In Poland there is really no other reason than slight differences between speed limtliters callibration (+-2kph). No uphills on Polish highways, no other variables, just that one. 2 minutes of trucks overtaking each other on 2-lane is still very common view even though there are more and more places when overtaking is explicitly prohibited for trucks only to prevent these situations from happening in critical areas.
Also the speed difference is much higher on Polish highways because cars can legally go as fast as 140kph while upper limit for trucks is 90kph. This is a huge difference, lots of brakes to burn before you slow down to match that speed.
