thats because thats when they hand in the recites to be paid its not done right here right now. mastercard or visa ( i dont know what MnT uses) but they approve them first not the bank.
also you can have a transaction out there for a year and they will still be honored like lets say you buy something from a “ma and pa” corner store that dont deal with debit cards too much and hand in the recites like once a month or once every 2 months and if you dont have money in the account its automaticly honored and comes out of your account weither you have it there or not its going to leave you with a neg balance and now the NSF fee.