I found this:
"Some merchants authorize for more than the amount of your purchase. In certain industries, such as gasoline, restaurant, hotel, etc., the business standard may be to over authorize by $10 or $50, or by some other pre-designated amount, in order to ensure that the necessary funds to cover your purchase are available. After you've made your purchase, the merchant will confirm the actual amount within a few business days of the transaction. Upon receipt of the confirmation, your available funds will be adjusted to reflect the appropriate available funds.
For example: A gas station may over authorize your Card for $50 (usually only at pay-at-the-pump stations) even if you only pump $10 worth of gasoline. This will reduce your available funds by $50 for a short period of time. Within a few business days, the merchant will send the confirmation of your purchase for only $10. Upon receipt of that confirmation, your available funds would increase by the $40 that was being held by the $50 over authorization.
It is recommended that when using your prepaid Card for gasoline that you pay inside the station rather than use the pay-at-the-pump in order to avoid this type of over-authorization."