You have a dipstick with markings at .50 .58 and .67. It looks like you are pretending you can measure the amount of consumption to an accuracy of 1/100 of a liter.
The graph shows measurements at .55 .56 58 .59 .60 etc.
These measurements are obviously fictional numbers. Measurements to 1/100 of a liter are well beyond the ability of anyone to accurately measure with a dipstick.
And one can't assume errors in measuring will average out unless there is good reason to believe that errors are randomly distributed. There is good reason to believe the errors are not at all random here.
There is also evidence of faulty arithmetic. There are two measurements that indicate .63 liters low on the dipstick. One of them was for a driving interval 1031 miles and the other was measured after driving 1203 miles. One can only wonder what sort of new math was used to arrive at the conclusion that the "extrapolated miles per quart" are the same for both those two intervals.
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