Ours is similar, due on the 25th but usually the 28th or even the 30th before the transaction takes place, not sure it there is any reason behind it. In saying that when we asked to change the due date with our mortgage provider from the 25th to 27th (pay day) they told us it couldn't be done.
same story with NIB, also depends if the the date falls on a weekend, then it moves it to the monday.
I asked for the 5th because I get paid at the end of the month and it gave me enough time to get it from an external bank to NIB. But is seems to actually happen between the 7 and 9th.