instrument cluster MPG readings

[omodos]

hi everyone, can someone please let me know what I have done wrong?
over the weekend I did an amp draw test and also disconnected battery, connected all back up as per correct procedure, well my OBC went back into locked mode, so seems the battery removal did this as well as reset date and time (funnily enough not preset radio stations!)

Previously the mpg reading I got was about 29.0MPG when pressing the stalk button to cycle through the options I had which I set as mpg and range (didn't want the others like start time arrival time distance etc)

Now been 2 days since this, I have done some town driving and am getting 21mpg! reading, and this reading is changing real time depending on driving style , whereas before the 29.0mpg was constant and would pop up to 30mpg on occasion if I did some efficient motorway driving....

So is it that I haven't driven enough for the OBC to gather data, having said that I have not predefined a start arrival time or distance or speed for it to do so hmmmmm and neither did I do this previously