Fuel consumption mpg value on dash jumping up and down

[srsttrans]

Hello everyone. Volvo VNL780 Cummins ISX 15 CM2350. 490K miles. Fuel consumption value mpg on dash jumping up and down no matter if you driving downhill or flat road. Value can go up to 1 mpg. For example if going on flat road pedal or cruise mpg value will jump 5.9-6.3-5.8-6.2-6.9-6.3. I thought it may be fuel pressure sensor on the rail but checked reading on the other truck and reading are the same. Could it be injectors or anything else? How is this value (mpg on dash) being calculated? Truck starts fine - do not experience long cranking. Fuel miles not great but ok. Its been always getting 6.3. Didn't see any bubbles in Davco filter, but still need to check in return line. There is Injector test in Insite 8.3 which passed ok. Any suggestions?