Like jjblbi stated, the MPG read-out apparently can be calibrated, although, I was not aware. As others have said, it is just a good indicator of what's going on. The MPG read-out in my 2011 Tiguan gasser is a smidgen less than hard numbers tank after tank.
My practice has been to fill-up to the max, vented at each fill-up. I have set-up a rather complex Excel Spreadsheet for fuel as well as every other expense. So, for example, I can look at the individual MPG of each tank, average and weighted average. My 2000 Jetta had a running average of just slightly over 51 MPGs. That's all driving circumstances combined. My son owns it now. I have the same spreadsheet for my 03 Jetta.
My practice has always been to buy fuel wherever it is the cheapest but, I do not drive out of the way to save 2 cents. As stated, I just fill until it will not hold anymore, vented. I've never experienced any "bad" or contaminated fuel. I've been driving VW diesels for 44 years ........ likely close to 1.5 million miles! My first VW diesel was a 1980 Rabbit with the little non-turbo 1.5 engine and 4-speed transmission. I never could break 50 MPG with it.