I over thought it for a long time. Maybe you are too. It is the common use of the word "volume" meaning an amount of enclosed space. One volume of CO2 in a Corny keg is the amount of volume that will fit in the volume of a Corny keg. The fun thing is that because there is space between molecules, that it really doesn't matter whether there is anything else in the keg - like beer. It still holds the same amount of CO2 at one volume. Now the reason why we use this description of how much gas will fit into a given volume, is that if we have more than normal pressure - say twice, for example - then we can get two volumes into the same keg. So two volumes is the same as saying twice atmospheric pressure. That is easy because it doesn't rely on you knowing what the atmospheric pressure of CO2 is.