Yes, Kent, the problem is your confusion, not your camera's confusion.
Originally Posted by Max Power
Reflective light meters are very, very simple beasts. They do not know what they are looking at. There is no computer. What they *all* do (with the exception of very modern meters, see below) is measure the luminance of the area of meter coverage (some "weight" one part of the coverage area more than another, but don't worry about this for this discussion).
The meter then tells you what exposure you need to set to make that luminance record as 18% gray on film. It doesn't matter if you point the camera or meter at a black card, gray card, or fresh sunlit snow, the result is the same. The exposure calculated is the exposure needed to record that amount of light as 18% gray on film. (The resulting *action* is the same, but the reported *exposure* recommendations will be very different in each case.)
In camera meters *usually* work because most average scenes that the average snapshot shooter points a camera at, typically averages out to 18% gray over all the values in the scene.
But if you happen to go out in the snow, for example, you'll find that if you trust your meter (and then let KMart process the result) your snow will render as 18% gray, not the brilliant white you expected.
Regarding the one stop difference you found between the black and the gray card, that could be for a variety of reasons. The lighting may not have been exactly the same. The angle of the meter to the target may have been different. The black card may not have been truly black. But with careful measurement you should have found that when metering the black card, both meters should have indicated an exposure several stops more open (or slower shutter speed depending on how the meter works) than the gray.
The only differences have happened recently with the very newest cameras, which add processing to the analyzed image. New Nikons actually compare the image being metered to a database of similar images, and recommend exposure settings similar to those images. Amazing and something I will forever avoid. Perhaps someday new cameras will detect where you are standing with GPS, where you are pointing with an electronic compass, realize you are in Yosemite National Park, and just record one of Ansel Adam's images. (That's what you were going for anyway, right? :-)
Edited to add the following:
ps: While the meter's operation is very simple, this simple but precisely predictable behavior of the meter is critical to calculating proper exposures for serious photographers. Anyone using the Zone System or similar systems depends on the meter calculating what exposure will make a particular spot in the scene as 18% gray. They can then adjust their exposure to "place" that spot on the value they choose. Typically using the Zone System, a photographer will measure a shadow area that they want to represent as their minimum area of detail in the image, then reduce the meter's exposure value by 2, 2.5, or perhaps 3 stops to "place" that shadow area into "Zone II or III", which is significantly darker than middle gray.