Yes, sometimes merchants send an authorization before they have the final total. For instance, gas stations often authorize a set amount before you pump your fuel. Or, if you add a tip at a restaurant, the pending transaction will usually show the pre-tip total, while the transaction that posts will reflect the full charge.
If the amount isn't correct when the transaction posts, contact the merchant and ask for the error to be corrected. If the merchant refuses, we may be able to help you dispute the transaction.