This is paraphrased from last time it came up so I apologize for any inaccuracy: They're reject cells from the manufacturing process. In manufacturing you have

This is paraphrased from last time it came up so I apologize for any inaccuracy: They're reject cells from the manufacturing process. In manufacturing you have a specification your products must meet or exceed for you to sell them with a certain rating. Some fraction of the cells a factory produces don't meet that specification, so they're either thrown out or used in unimportant applications like this.