Boy, you got me on this one. I THINK it might have to do with the fact that the later you are in the memory slot order (0-3), the lower or "looser" the timings (CAS latency) are. So, you probably do have a defective RAM module (doesn't run at the listed spec), but it doesn't matter with the looser timings in slot 2-3 (or 3-4 if you're not calling slot 1 slot 0, which is actually how they're listed). That's my guess at least.
They should have replaced the module, but got away with giving you a workaround instead. That's ASUS for ya. Great hardware designs, TERRIBLE support and terrible software engineers.