Atari found out, that DRAMs from particular series "remember" bits for a long time after the power is off. And if you switch power off and on to force cold start, the computer boots with garbage on screen, then hangs up. So they developed a fix: mix each set of DRAMs with a single chip from another batch, the "fast forgetting" one, and the assembled computer passes tests OK! It was easy and cheap, so they did it.
Can't see anything unbelievable in such a story. It is recommended to add one or two different chips when replacing defective RAM in XL/XE's, it's common practice actually. And I didn't make it up, it came from guys good at electronics.