Hmm, i was just thinking about this, I also have the same problem, but mines even worse because I mix my own fuel, E-42.5, so I never fill it up to the same levels :S. Not to mention my fuel gauge hasn't moved since my last boost tuning run.
Anyways, the problem i see is if you fill up with gas in the morning, it's colder-denser, the pump measures volume, but the engine injects it by mass, (1g of gas does 1g of gas's work.) So if you fill up one day when its cold, and one day when it's hot you could have a noticeable difference in the actual amount of gas your getting!
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