Self service gas stations can have one of the following two ways of processing payments:
The terminal at the gas station takes a set amount of money from your account, then adjusts the sum accordingly to the amount of gas you used right after you’re done.
The terminal will take a set amount of money from your account, but only after some time (usually 1-3 days) will the data with the actual amount used be sent.
Once this data is sent, the amount is adjusted.
So, if the gas station processes the charge according to the second option, you will be charged more initially, but the money will be returned to you within a few days.