Because the level sensor changes the fuel level in the tank slowly while you're on the hill, and then the level drops when you're on the level again. Or, the other way around, depending on which direction the fuel level meter is installed.
The situation can be remedied by re-installing the fuel pump so that the lever is located to the side of the tank, rather than the front or rear, but the pump won't always fit in this way.
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"¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"
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