Memory modules with mismatched or oddly marked chips ("PC-133" instead of real marks) may contain used parts that haven't been tested very thoroughly. They may have been assembled in a sweatshop where the workers aren't paid enough to care, and the only testing done was with a $1,000 machine that's barely better than a software diagnostic.
Of course, then there's PNY memory, which may not be much better, judging from the module I received that contained remnants of a solder short across the fingers. The CompUSA manager didn't understand why that was a flaw.