There is 12 oz of Freon , ie R134A in this 12 oz Can below, right?

On another website I was told that are actually only 10 oz of Freon in a 12 oz can as shown below ( please disregard the Brand Name, the picture is show only to depict can size )
I am preparing to recharge a non-Mercedes Diesel AC system after rebuilding the compressor myself.
Honestly I think the guy who told me this, who is very very sharp and good at electrical issues, is perpetuating a urban legend. I would believe him if the can was loaded with Dye, Oil sealer and it said so, but in my book a 12 oz can of Freon contains 12 oz of Freon. This isn't the first time I have run into someone who is sharp and yet unwittingly perpetuates an old wives tale.



( please disregard the Brand Name, the picture is show only to depict can size )