This is an old question, but as someone already gave an answer yesterday, here I go...
My car doesn't have a milage display, and it also doesn't communicate any information about its current fuel consumption via OBD. (I also don't know if this is foreseen in the generic part of OBD in general.)
It seems, the app "Torque" takes the RPM, the intake air temperature, intake air pressure (all available via OBD) and the displacement (entered by the user) to calculate the air flow through the motor. Assuming a perfect air/gas mixture, the amount of fuel that could be burned is derived, and with the speed via OBD/GPS, the current milage is calculated.
However, this milage is not always correct. Driving downhill, the current consumption should be zero due to fuel cut-off, but the app shows a significant number. Pressing the clutch, fuel has to be burnt to keep the motor running, which leads to a very low consumption. Instead, the consumption shown by the app decreases.
This leads to the conclusion that the app doesn't know anything about fuel cut-off. It just knows how much air is going through the motor and thinks the consumption must be higher at the higher RPM.
Another scenario one can imagine is that the app doesn't display the higher consumption shortly after starting a cold engine.
I don't have experience with this app together with other cars, but it seems it uses the most suitable sensor data. And when it's not available, it tries to derive this data from other sensor data. But this could lead to incorrect results. (Yet, the app does its best to display the milage)