IBC - International Bank Of Commerce Overdrawn fee with no reasonable explanation McAllen Texas

I checked my account over the web page and found out that I was charged 4 overdraft fees of $25 each. Three of the fees are reasonable because I ended up with a negative balance, but one of them has no reason because I had a positive balance after the deduction. I talked to a representative from the bank and he told me that there was a pending transaction of $15.01 that was deducted; therefore, in their system I had a negative $10 balance which does not appear on the balance that I have in the web. Apparently, all the overdraft fees were from October 27, but the pending transaction of $15.01 was not paid until October 30.

I don't understand why is it that they had to deduct the amount of $15.01 way before the bank had made the payment to them. I have a copy of the account activity from the web, which shows that I had a balance of $4.53 after the $30.00 item for which I was charged $25. I even told the representative that why they did not deducted the amount on that day for me to see a negative balance, but he told me that because the bank did not paid that amount until the 30th. Please tell me what I can do, to get a more valid answer about this problem. Thank you.

Alma
La Joya, Texas
U.S.A.