That is probably similar to how you could use both sides of a single sided floppy. Both sides were tested and the factory ended up with a huge pile of floppies that passed both sides and a small pile that failed one side (and some that failed both sides and were discarded). The market wanted more single sided disks so they simply moved a number of floppies from the first pile to the second. That meant that you mostly likely got a double sided product when you paid for a single sided one. But it was a risk.
Same thing of processor and memory speed bins.