SSDs have different partition sizes, because manufactures reserve spare blocks, for performance and to help stop the drive from wearing out prematurely. Different companies have different ideas on how much space they should reserve.
As you use your flash drive, blocks will eventually go bad, once they do it will use a spare block (from the blocks outside of the partition). When all the spare blocks are gone; blocks will be marked as bad an unwritable, until eventually you can not write anything to the SSD
at all. You will still be able to read the data though (ideally).
Also many controllers use this space outside the partition for wear leveling and fast writes.