They got two new ones.

But it would be very weird if different drives in the same batch had different capacities. That's a lot of wasted effort.

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.

Yep. This is very common and is called binning:

https://en.m.wikipedia.org/wiki/Product_binning

But they were in the same bin.

Right, but all that means is they met the minimum standards for that bin and didn’t meet the minimum standards for the next one.

As far as I can see the author now owns 4 memory sticks, but only looked at the flash on 2 of them.