500ml is a fraction, but it is a half not a third.

Now you can get an approximation to a third using the metric system but for some reason engineers don't do it with linear measure 'cos the thought process is gone.

Now I have seen 33cl used in Europe but I suspect that it was sales people who thought that up.

Another phenomenon that comes hand in hand with decimal fractions is this modern love affair with excessive accuracy. When the calculator took over from the slide rule folks seemd to think that ten decimal places were required for all measurements.