Thank you very much for your reply, Pavel!
I now understand that the on-hand/required percentage is not as accurate due to the way it is calculated.
But the percentage in the "fuel received"-line I am still not able to construe, I'm afraid. I have attached a screenshot of the Unit Supply Detail window of the unit I am referring to (sorry, it's the games's German version, in English it says "fuel received" instead of "Treibstoff erhalten").
Am I mistaken about the way the 4% are calculated? It received 144 fuel, 161 is the total requirement, at the end of the previous turn it had 8 fuel remaining, resulting in a demand for 152, I'd suppose. (The previous turn it had a total requirement of 155 fuel, by the way. So it obviously received some replacements which increased the total the requirement for fuel, but not that much). The demanded 152 fuel were reduced to 144 fuel actually delivered to the unit because of the the Axis Rail Supply modifier, I would say. But unfortunately I cannot get a grasp of what these mysterious 4% mean :-). As I said, I would assume that this percentage expresses supplies received (=144) against supplies required (=161) which was 89,44%.
Apparently I am missing the forest through the trees here... So I would really appreciate your explanations :-)