Some people have to budget how much gas they can fill at once. Maybe the budget allows for $30 a week, so they stop at $30, or in OP’s case something random like $28.73.

Alternatively, if you prepay at the counter, instead of handing them an even $30, you could hand them $29.39

Good job lad

Relevant xkcd

The underpaid lab assistant who quietly fixed all the others problems caused by the misuse of lab equipment.