LonardoSecundo,
May I suggest an addition to your remarks here. You must consider the accuracy of the report and the precision of the report. You add them to get the maximum error band. The precision is how many decimal places are presented. The accuracy is related to the accuracy of the data before transmission.
The precision of the raw data is two decimal places, 1/100 of a degree. This is shown on page 46 of the English report and is not shown on the corresponding list on page 49 of the French report. 1/100 of a degree is on the order of 0.6 nautical miles.
The English language report simply declares, "The position transmitted was the aircraft’s FM position which, in normal conditions, is close to the GPS position."
You stated, "The maximum error of the accuracy of the values of position given by the BEA is 0.5 tenths degree, in one way or the other."
That would be on the order of 5 times the 1/100th degree report precision or about 3 nautical miles. Adding the two, which is conservative since we do not know if the position was truncated or rounded in one direction or the other, we get about 3.6 nautical miles. That places the plane on its proper line of flight within the total accuracy we can expect.
Supposing you misstated what you meant the results are different. The accuracy before rounding of the FM position may be close to the 1/1000th of a second GPS reports or even the 100 meters or more the plane FM rounding would not materially degrade the precision of the report. That would mean they're about 2.3 nautical miles left of track. They may have started a turn a few seconds before the report. Or they may have already been in trouble of some sort.
JD-EE