Not strange at all; first they make the batch, then they test them to see how many of the bits are actually writable, and sort them according to how well they turned out.
This is also how phone screens are made (a huge panel is produced, then lit up; some pixels are stuck, but phone-screen-sized pieces can be cut out of it where there's enough good pixels).
This is also how most CPUs and GPU models are made - test each sub-component, disable the parts that fail testing, then price the result according to how much of it works.
