PowerBook G4 1.5GHz 12" - memory RAM error code: post/0/2048DIMM1/J31

I have a 12" PowerBook G4 1.5GHz (Model: M960LL/A) that I am getting ready to sell to a friend of mine. I had an additional 512MB of RAM installed in addition to the 256MB that is built in to the Logic Board. To make sure everything was good to go, I decided to run the hardware test. The memory portion seemed to not complete/pass and I received an error code: "2MEM/103/4:DIMM1/J31" so I figured I would fix the problem by replacing/upgrading the RAM with a 1GB stick.

After installing the new 1GB RAM, I ran the hardware test again. The entire test passed/completed this time, but I still received the error code "post/0/2048DIMM1/J31". I reseated the RAM and got the same result.

Also, System Profiler shows that I only have 768MB installed, which isn't true. It should say that I have 1280MB. This leads me to believe it's the actual RAM slot that is bad? But I don't know.

Okay, I just put the old 512MB stick back in and reran the hardware test. Everything passed!

So I guess my question is why the hardware test failed the original stick once, then passed it the second time?

And why the new 1GB is only showing up as 512MB?