I think it is because the level of fuel in the tank drops vertically (obviously!) but the variable resistor/potentiometer is rotary. Visualise a point on a circle at '12 o'clock' moving clockwise to '6 o'clock', from 12 to 2 the vertical motion is not that much, then from 2 to 4 much more, then from 4 to 6 not much again, all for the same rotary movement. So a small drop in level results in a larger rotary motion; hence gauge drops quicker. Also could be to variation in tank shape.
I've never had accurate fuel gauges and in aircraft, where you would think that they really should be accurate, far from it. We use US gallons per hour burn and allow for large reserve, you can't just pull over at 2 500 feet if you run out of Avgas!!!
Sorry for being semi-technical but you did ask.