I find it odd that the Belden wire tests nearly 1/3 lower in resistance that is claimed for it. The claimed resistance is essentially standard for 12 AWG wire. I can understand a reputable brand testing a little better because the target size has to be a little bigger to ensure that normal manufacturing variability does not result in a substandard wire, but that much bigger doesn't make sense. I think there are some problems with how it was measured. That said, assuming the testing was consistent, I think the results are meaningful, particularly the ones that are well above standard.
There is more going on with the Fry's and Best Buy wire than being aluminum. The difference in conductivity between copper and aluminum doesn't account for the difference. The cross-sectional area of the wire must be smaller, too, or they are using some weird alloy. This is not a problem just with these two retailers. It is a problem at Home Depot, Lowes, Walmart, etc. If the label on the spool doesn't say copper, it probably isn't. Beware of familiar consumer brands like RCA, GE. When it comes to speaker wire, they aren't the same brands your father knew.