Yosimite Detects Faulty RAM Module but Apple Hardware Test Does not?

I just bought a used MacPro3,1 on Amazon. The system came with Mac OS X 10.6 but I downloaded Yosimtie and it states that one of the 2GB modules is faulty.

When I installed Yosemite it wiped out diagnostics for Apple Hardware Test (AHT) but this isn't a big deal as it can be downloaded and booted from USB for particular Mac models. When I ran the AHT in extended testing mode (a test that runs for two hours) it states that I don't have any issues: