I suspect the reason why no one has responded to this thread is because no one has direct access to the inter workings of all of these programs.
Also, different software will have different ways of doing things. I can only speak to ColorThink Pro, so I will limit my comments to that. ColorThink Pro calculates gamut volume in conjunction with its grapher. By default the grapher will plot the proofing direction of a profile so that you are able to see the gamut of the device, what the plotted device is capable of reproducing. We think this is what most people are looking for when they want to plot a printer gamut for example. Other graphing software will draw the gamut of what a certain profile will give you when printed
. This is not the same as giving you what the printer is capable of
. For this reason, you might find ColorThink Pro giving you a slightly larger gamut volume number than those that calculate a rendered gamut volume. You'll find more information about this here:http://www.colorwiki.com/wiki/Color_Management_Myths_26-28
If you are going to look for the rendered gamut volume, you have more questions to ask: The rendering intent used and the source colorspace both have an influence on how large the gamut of your profile will be, so you need to include this information when comparing.
While the ColorThink Pro gamut calculation should be quite accurate, it is not intended to be some kind of lab-reference for determining absolute volumes. It is intended to be a quick way to bring up quantifiable relative volume numbers for comparison between profiles. The accuracy should be high for properly-built, normally behaving profiles like the canned profile in your example - rather than for those with strange twists, folds, spikes or large gaps in the gamut.