Amount reserved but not deducted from balance, how does that work

My daughter moved from Capitec to FNB. I opened the Capitec savings for her because she was still in school but when she started banking she felt that she needed a "grown up" account and opened one with FNB. I use to work for FNB and it is a good reliable bank, well after all her experiences I am not so sure. I think they do not have experienced staff working there anymore. I have hear from a few clients of FNB that sometimes when you pay with your debit card the money gets reserved but only gets deducted a few days later. I have never heard that the transaction gets "reserved" but the purchased amount is not deducted from your balance and only gets deducted when the purchase show on your statement a few days later. This happened a few times now with my daughter. So what i would like to know why "reserve" the transaction if your IT system is not deducting the amount from your balance? That shows you have a serious issue with your operating system. I told my daughter to close her account and move it. I tried reasoning with them and it is useless, they know their system has a default and clearly admits it .......