MPG from dash and Fuely are not the same

[kd8tzc]

Just curious how this all works. I have been fuelly for a few months now, and fuelly is always a bit lower than my estimated MPG that the car displays on the dash. I might see 32.1 MPG on the dash, but Fuelly will tell me 29.0 mpg. I could see it being slightly different (32.1 vs 31.8) but it has been a bit more than slightly. What is causing this and is there something I may be doing wrong? I always fill up the car completely each time.

Thanks