Unpaid item charges?????

Now listen to this.... this is what they send me!!! " At the time when the purchases were done, there were sufficient funds available in your account; however at the time when the merchant approached the bank to release the reserved funds there was unfortunately not sufficient funds to cover the purchases hence the honoring fee was charged by the bank. Please note that a great many merchants do not bank/present for settlement daily and will only do so every second or third day and maybe even longer in the case of a weekend. In some instances smaller card purchase will not dial through to the bank for authorisation (depending on the merchant’s floor limit) and the bank would therefore be completely unaware of the purchase until the merchant presents it for settlement. Unfortunately the onus lies with a customer to keep track of their spend, debit orders and card purchases accordingly in order to ensure that they have sufficient funds for their monthly commitments and in so doing avoid items being dishonoured and any associated penalty charges. THEY CAN'T ANSWER THIS QUESTION I POSED TO THEM: " Why take my money from my account on the day of purchase then, AND if I did not have sufficient funds in my account when the merchant presented the payment, WHY did my account not go into overdraft???? """