Actually, it may be not so much the nature of electricity as geometry
Those older cars had the float swinging on a pivot -- with obvious geometric effects on the way one inch of lowering changes the effect on the sender signal at different levels; a lot of the newer ones with linear markings on the gauges have the float sliding straight up and down a shaft. As I remember, most older Ford cars with pivoting sender floats had the same pattern of the guage dropping faster after the mid-level readings.
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