Hello,
all Sandisk drives and other manufacturers have spare blocks that can be used if some fails, for Sandisk there are 5-10% of the maximum capacity in order to keep the drive safe for a long time. For the extreme pro there are more blocks to spare and thats visible to the capacity of the drive also, thats why its counted so strange.
Of course you can let more space available for over provisioning if u left unallocated space to your drive that will then be used if 0on more spare blocks are left to replace.
