Why does my car constantly give me different ranges on a full tank?

My 2016 Ford EcoSport 1.5 TDCi said I had 538 miles until the tank was empty when I first filled it. In the summer, it said 600 miles to empty. Tonight, I filled it to the brim and it said 508 miles to empty. Have you any ideas why there is such a difference in the mileage I'm getting?

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The estimate is based on your current mpg extrapolated. So if you are driving very economically it will estimate a greater range than if you aren't.
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