The answer to this is that all .resx files (all resources, for that matter) are compiled by VS into a single library (.dll) or satellite assembly, which is always named after the main assembly. Makes sense once you realize what's happening.
My original post happened while I had only a single .resx file, which I innocently named the same as the main assembly. Since the complied satellite assembly *seemed* to be named after it, I then expected that changing the .resx file's name would also change the satellite assembly's.
Live long, write lots of code, and prosper.