Fuel gauges aren't that accurate.
Even on my 2010 VW a full tank takes me about 400 miles, yet it always shows half a tank after 300 miles.
And we all know how trustworthy VW are with gas mileage
If you get an accurate mpg you can figure out if you have a problem or not.
Stop start town driving will give you worse fuel economy. Riding style will also have an effect.
Fill your tank to the top and make a note of your mileage.
Then go for a long ride until the tank's nearly empty.
Fill your tank to the top again and make a note of the amount of gas you put in and your mileage again.
Use these to calculate your actual MPG