Well there is an answer in Your question. Device is estimating. It is estimating depending on information You provide when configuring GNSS Fuel Counter. Please see here: https://wiki.teltonika-gps.com/view/FMB920_Features_settings#GNSS_Fuel_Counter
Sensors can help You to get more precise information regarding fuel level. While GNSS counter estimates consumption on conditions You provide and average speeds, but that does not mean that the driver habits will always be the same like configured on the device and driver can drive faster than average speed configured. So there can be deviations depending on driving habits. And this counter more or less estimates how much fuel was consumed.
But using sensors You can calculate how much fuel was actually consumed more precisely for example.
If there are more questions regarding this, please ask.