The smaller capacities are shown because of the over-provisioning. The varying capacities are partially explained by this, and partially explained by what is presumably the outcome of lower quality flash memory and internal requirements for product consistency. If you run a benchtest on each drive, or even more so if you simply time file transfer times using your own files, you are most likely going to see substantial variations in product performance.
from another thread
This is completely normal, but with the way companies advertise, it is a frustrating practice. The missing space is explained by over-provisioning , and the purpose of this is for health & speed benefits.
With flash media (SSDs, SD Cards, USB flash drives, etc.), there is a tiny microprocessor inside the flash drive that controls the NAND flash itself (i.e., the flash media that stores the 0s and 1s using electrical impulses). One task this processor takes on is to ensure data is being saved to “healthy” NAND, and to take “sick” NAND out of use to avoid massive data corruption. In order to accomplish this, the flash drive needs to have spare unused space that it can use to replace any blocks it retires from service. So if a drive “retires” 20 MB of space due to bad blocks, it needs to have a spare 20 MB to replace it with & that 20MB will come from those 2GB that you cannot see. Additionally, some flash drives will use this reserved space as a means to increase file transfer speeds in several different ways. So while your drive is indeed 64 GB, the 2GB are not showing because they have been allocated to the flash drive’s microprocessor.
Now with that said, the way companies handle this is kinda crappy. If they advertise a product as being a certain size, they should either disclose what the usable size is, or simply make the drive a little larger so that the end user will get 64 GB that they themselves can use (so the size of a 64GB flash drive would actually be 66-68 GB to give the user 64GB to work with. But very few companies do this. If you have an iPhone, take a look at the actual size versus the advertised size. My 64GB iPhone 6s has about 55GB of usable space–a difference explained by over-provisioning.