In my opinion, it was never intended that armies of under 500 points should not get a false lead. Hence, every 500 points is read as, "each 500 points (or fraction thereof)."
Breaking it down, one could not argue the following sentence "For every part, there is a counterpart," to not mean anything other than, "for each one, there is one."
The same applies here: "For every 500, there is one." That means that if there is more than one 500, but less than two 500's, there is two, etc.
Think of each group of 500 as its own, individual group.