Gas reading

#1
Weird question. Not a big deal, just curious. I have a 2018 le. I bought the car in March. Every Tim I'd fill up the gas tank, the dash read 386 miles. Once I really topped it off and got 403 miles.... now for some reason the last 4-5 times of filling up it will read 322 (or somewhere around there) I don't understand why the difference?
 
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