All the profiles (using either SW) produce profiles that have slightly smaller gamut volume than the ones provided by printer or paper manufacturer (for the same printer/paper combo). The size difference is a fraction of a % to maybe 3%. Is this expected?
It's normal that no two print/measure cycles using different equipment will perfectly match. Hard to know exactly why such differences occur, and a factor may be the profiling software itself, as well as how you are viewing the gamut volume.
i.e. one wonders if the paper and ink and printer "at the factory" are particularly good examples, and what the dry down time is. They may be using different instruments that are calibrated slightly differently or have different illuminants (UV content).
The way in which the profiling software determines the white point may be different.
The white point chromatic adaptation matrix may be different (ICC spec. uses "wrong" Von Kries, Argyll uses the more visually accurate Bradford matrix.)
Whether you are examining the absolute or white point relative gamut may make a difference.
etc.
Am I correct to assume that GV alone does not indicate which profile is better in a given situation, and that one needs to consider the image and how a given profile interprets the image?
Checking gamut volume may be a sanity check, but ultimately I don't see it as something to worry about. When you make a profile it reflects the actual color of your actual printer with the best instrument you have available to you. You have therefore put yourself in the best practical position to be making good prints.
Many other considerations will likely have a greater influence on the subjective result, such as the way a photo was shot, the way it has been adapted from the scene reality to the limitations of display and then print media (luminance/dynamic range, viewing conditions and gamut limits), etc.
