Charged extra for an amount already reserved

There is thing that I dont understand that FNB does and it still doesn't make sense even after they explain. When I swipe or make an online purchase an amount is reserved right and also debited from my account meaning the purchase has been successful and made. Then this amount is no longer in my account it will either be in my pending payments or history transactions, please remember I don't have the amount anymore. I have called several times about this issue but I see it still happens every month.

Ok so they claim some merchants or retailers bank or claim the amounts late. Then what they do is when that merchant or retailer claims or banks they will charge me for making the payment for me without the funds in my account yet the amount was debited and reserved on the day, hour, minute or second of purchase. My question is and always will be. Why am I being charged for that as a payment done on my behalf when my account didn't have yet the amount was debited and reserved already? I am really not happy and very disappointed. My account doesn't have a credit or overdraft facility yet I will always find my acc overdrawn with R200 per transation on an amount that was reserved and remained pending. My idea is the amount has been reserved already so why am I being charged. Even if the merchant banks late that's not my problem cause the amount has been debited or deducted and placed in pending. FNB PLEASE explain.