You didn't just type that out, or did you ?
If what you state is true then the fuel levy would never have gone up as the price per liter went up.
What you stated is absolutely false, the fuel levy is calculated at a tax % level per liter. Each time the fuel price goes up, the tax levy amount of the increase also goes up. I guess that is just an accident and have nothing to do with % and math.
It doesn't matter if you want to call it a "fixed" fee or not, at the end of the day when the fuel price goes up then they request the fuel levy also to go up. They have been doing it for decades now, and it works out no different to a % per liter. The funny thing is that when the price dropped they still requested a levy increase per liter.
I wonder why we don't have 10c per liter fuel levy these days, guess you should remind the government about that.