Water is pretty much incompressible, so there's a good chance the internal volume is two litres.
0.3m^3 is 300 litres. So, I'd agree with it being compressed by a factor of 150. The 300 litres will refer to some form of standard conditions, a specific temperature and pressure though I wouldn't like to guess what.
I imagine the discrepancy follows from measuring 300 litres at one temperature and 200 bar at another. Perhaps 300 litres at room temperature, but 200 bar at whatever temperature the argon reaches as it is forced under pressure into a container?
I base this on the belief that argon is probably forced into the cyclinder in a brief period of time, and that the only sane way of measuring the amount of gas that's gone in would be pressure.
Or you've got air in the cylinder, as well as argon. Or the 200 bar refers to the rating of the cylinder, not to the contents.
edit: A decent third choice is that the perfect gas laws don't apply terribly well to argon. I'm too lazy to research this.
edit2: After a couple of minutes thought, I think the most likely explanation is that the perfect gas laws don't apply perfectly. PV/T isn't always a constant. There's no good reason to print a pressure rating on the cylinder, so what you've probably got is 300 litres of argon at "room" temperature, compressed to 200 bar.
If cylinder size is 2L (0.002m^3), is 0.3m^3 the unpressurised volume of the argon? Doesn't seem to fit with PV/T = PV/T.
