I expect the MPG on the dash display to differ. I suspect, unless there is manufacturer skulduggery, for some to to off high and some low. After one fill up on my car my dash indicator is low. I will know more definitively after more fill ups.
I have done 18 fill-ups so far and found them to be close. Only a couple tenths of a gallon per mile off. Sometime high, sometimes low and sometime exactly the same. Hence I think it is a reliable indication.