Output files are named after the layer groups, not the individual layers. So if you have a group with "component, Aground, Dground", it gets saved as "group0" or whatnot. The default builds allow up to eight groups.
In the HID pcb, I changed this to be "copper0" instead of "group0" so the name was more obvious. I might change it to "topcopper", "botcopper", and "in3copper" (example) based on the component/solder groupings. Opinions welcome ;-) If your question was "why is it spitting out this layer I'm not using?" then you'll have to use something like gerbv to view the gerber and see what's on it.
