When buying a top-up, you can see the final amount that will be sent to the phone number in the order summary.

If the phone number has received a different amount, it may be due to one of the following reasons:

• Some operators offer the option to advance credit when the customer doesn't have sufficient funds. After receiving a top-up, they may deduct this credit from the balance to clear the owed amount.
• If you have sent the top-up during an operator promotion, the bonus might be applied or claimed in a different way. 
• The country you are sending your top-up to might apply a sales tax.