These aftermarket systems can’t see how drivers conserve fuel, they do so by being off the throttle far longer than others.
Junction half a mile ahead?
driver A is off the throttle letting it slow gradually then uses auxilliary brake to slow…but only just enough to hopefully not actually come to a halt, maintaining progress by timing the approach to not actually stop, if a roundabout and no one can possibly be baulked he straightens it out meaning he can take it 10/15mph faster than keeping in lane, do this early morning especially on dual carriageways with numerous roundabouts and the savings are significant.
driver B steams on till the last minute, then its brakes combined with auxilliary (i’ve known some switch the exhauster off 
) to quickly knock the speed off, then has to come to a complete halt because approach is ill timed, then due to very low speed more than likely has to stay in lane around the island.
billy big ■■■■■■■■ specialises in overtaking other lorries on roundabouts, meaning that both have to follow the lanes precisely and both negotiate the roundabout slower than otherwise, bbb is a plant pot.
You travel one long dual carriageway and see the fuel difference between the two techniques, between which there is barely any time difference.
The big user of fuel is getting a stationary fully loaded artic up to 55mph, try it for an experiment for yourself, set your trip meter to zero and accelerate to 55mph, unless its downhill the amount of fuel you use is staggering, think about 2.5 litres and you won’t be far out.
If driver A spends his day keeping the vehicle moving, and driver B is constantly heavy braking and having to get moving from stationary continually due to bad planning, then the difference will be a hell of a lot more than 1mpg.
This is only one scenario, there are dozens all day every day, how one driver deals with hills for example, does he use the hills to advantage or brake all the way down then have to go under power at the bottom where another driver might have approached the previous peak slower, let the down hill do the accelerating and if he times it right just lets it slip high enough over 55 for a few seconds to not gain an infringement but is several hundred yards further along before having to resume power.
Some lorries it might make better progress, avoiding a gearchange half way up the hill, if the driver selects one gear lower just before going back under power, doing so might mean a hill can be climbed in 11th where not doing so might mean a further gearchange and 5 mph less speed at the summit, again telematics can’t see why.
Timing of traffic lights, how many times have you been approaching lights on a dual or motorway junction on a trailing throttle in order to reach them at green, about 400 yards before the lights billy big ■■■■■■■■ comes hurtling by still on the limiter then stands on the brakes, has to harsh brake, and is thrashing the living daylights out of the lorry to pull away again…helpfully holding the brakes on full on red hot discs in order to transfer some friction material onto them, which is so good for the discs , meanwhile our steady approach driver is still off the throttle and coasts past billy.
All the above, none of the aftermarket systems will take into account because it can’t see the road and developing traffic like the driver can.
The driver who idles the vehicle warm has an efficient engine (that might well last a lot longer than one worked hard from cold) and he might well be idling to help the turbo(s) last indefinately, telematics can’t compute these reasons either.
Not suggesting you are driver B Nomi, far from it, this is just general observations.
Sometimes these differences in fuel can’t be explained.
Many years ago i worked with one of the most professional lorry drivers i’ve ever known, he taught me a hell of a lot, mostly forgotten 
, oddly enough i see his son, another driver, a couple of times a week, a chip off the old block there.
Anyway, i, being younger was a hot headed flyer, i had a 320 ■■■■■■■ same as him, my cruising speed was typically 70 where his was 60, yet i used to get 1 mpg more than him almost consistently, no telematics involved at that time (how did we manage 
), this was the boss but without the pointy shoes and gelled hair putting the figures up on the wall worked out the old way.
No one could understand it.
By the way, i like upsetting pointy shoe wearers 