system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were flash storages sold in doubles of the storage (1, 2, 4, 8... GB) and now there are SSDs with 240, 250, 480, 500... GB?
gpt
Those SSDs will, in most cases, have a round 'doubled number' of flash memory inside them. However, flash memory wears out, so the drive has a pool of 'spare memory' that it can use to replace the parts of the memory that wear out.

So that 240GB drive probably has 256GB of flash, but 16GB is set aside for later. That 250GB drive is cutting it fine with only 6GB spare: but these are design decisions based on how reliable the manufacturer thinks their flash memory is.