What you've just described is all absolutely normal behaviour for the device. The average mpg for these cars will be about 19mpg, so if yours has been slightly lower than that (ie 15.9) it suggests that you're either doing a higher proportion of 'town' driving or using quite high revs - or a bit of both.
15.9, although it's a little on the high side, is still within reasonable limits. If you take it on a track you'll get significantly less, for example.
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'03 Grey >>> '06 Black >>>'10 Silver >>> '16 Grey ^
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