You NEVER get the "advertised" storage for any media - SD card, hard drive, thumb drive, lawnmower gas tank.
Flash media always uses a significant amount of storage for the file structure, housekeeping, spare bits (some cards will mask out defective cells and remap them) and just general overhead. I just put a 200GB card into my Amazon Fire tablet; I think it showed free space as around 185GB right off the bat.
Nothing to worry about; it is what it is.