It's no worse than the French use of "huit jours" (lit. eight days) to mean a week, or the terminology for musical intervals where a one-note interval is called a "second". I'd guess that the origin might the same in each case: if you start at 1 and count up N times, you get to N+1.
ငါးမူးသား(nga mutha): Literally "five mutha", but in fact it is only four.