Mazda6 2.2 diesel - Range reading v trip reading

Hi all,

purchased a Mazda6 (2016) reg in December with 118,000km on clock
The car is an executive black model and apart from the sat nav working once in ten times it is running well

( am working with Mazda dealer with regards to the sat nav, so far they have upgraded the S/W but that didn’t improve on the issue - the issue being when you select the sat nav feature you just get the loading gear wheel symbol and eventually it resets the infotainment system even dropping phone calls - the next action is a new SD card, this is on order)
Reason am submitting a post is regards the odometer and the range value

When I go to the forecourt to top up I have been putting in 20 litres of diesel (costs about €32) resetting the trip reading and taking a note of the range the the on-board computer is reporting

For example topped up with 20 litres Friday night the range was reported as 437km so far I have driven 30km (town driving to and from work, four trips) and the range is now reporting a range value of 365km!

the fuel economy is reporting about 6.2- 7litres/100km by the way

is the range reading on the car accurate ? For example driving to work I go up a number of steep hills and the range can drop by 6km when I reach the summit!