It has to do with manufacturing quality. 

Each disk is checked for quality once rolling off the assembly line at the factory. From the manufacturing process, disks do NOT come out without errors. They all have a certain amount of errors. When tested, if a disk has errors above a threshold, then it is discarded. If only one out of the two surfaces has errors, then the disk is still used, disabling data writing to the damaged surface within firmware. Statistically speaking, when a disk surface is bad in a production batch, there is a good chance that the other surfaces are also poorer quality, despite meeting quality thresholds.

So for example, say we have a fully healthy hard drive with 2 disks, so 4 disk recording surfaces to store the data on, each with a 250GB storing capacity, then we have a 1TB capacity.

Now take that same batch of disks produced, but, upon testing, one disk has an unsable disk surface, therefore not to be utilized. The manufacturer programs the drive's firmware (sometimes they may not even physically install a reading head for that damaged surface), where we have 3x 250GB usable disk surfaces totalling a 750GB drive capacity.

That was an example for disks with a 250GB disk density. Over time, they have increased, to 500GB disk density, 1TB and so on.

Similarly, the same kind of quality checking takes place with flash chips that go into smartphones, SSDs, cameras, USB flash drives, SD Cards, etc.

The best quality flash chips go into smartphones, cameras, SSDs, as they are expected to have high quality. The lesser quality are purchased by "lower quality brands" and use them in their products (e.g. those cheap generic promotion USB flash drives that people give out for free, fake devices, etc).

Hardly nothing ever gets discarded.