A threshold limit of $50 has been set, but I can fill gas in some stations for more than $50. Why?

CardValet controls are only invoked during authorization of a transaction. In some cases, such as gas stations, a card may be tested for validity by doing a $1 preauthorization, and the actual transaction amount is charged to the card after the transaction.

Some gas stations will preauthorize for a maximum amount, example: $126, and some gas stations will check validating, example: $1. In the latter case, the actual transaction amount may exceed the limit, while in the former case, an authorization may be denied.