Why do microSD cards have less than advertised space?

If you check the fine print, you may find that 1M is defined as 1,000,000, not 1,048,576. And 1G is defined as 1,000,000,000, not 1,073,741,824

This has been a trick used by hard drive manufacturers for decades. And it appears that the memory card people have picked up the same trick from hanging out with their sleazy hard-drive marketing friends.