I would agree with TC , does seem very low duncan. Depends at what point you fill up (empty on guage?)
Remember theres accumulated average MPG so unless you specifically reset the average at fill up , the mpg average will be across all fill ups since you last reset.....
At the very least the average mpg (if reset at the last fillup) , should tally (roughly) with the tank size.
So for me
last tank 370 miles travelled at average 30 mpg. I filled up on empty (but not reserve)
370/30 = 12.33 gallons
12.33 x 4.54 = 56 litres which is roughly the expected tank size
in town driving , the GTI does not do well tbh for mpg. A good rr session will put your mind at rest