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.
Plus if you didn’t buy a brand name SD card, and it was pretty inexpensive, you may have a counterfeit card. These are everywhere. I once bought a 32gb card which was labeled 32gb. When I formatted it, it showed as being 32gb but it actually was only a 4gb card. I bought it off of eBay. The problem was that I couldn’t tell it was a counterfeit til I almost filled up this 4gb. When I went into gallery on my phone, rather than a thumbnail of the photo, it would show a black square with a lightning bolt in it.
I had to find a program that would write and read to each sector of the card. I found out that I had another bogus card. The 64gb card I thought was good was actually a 4gb card. Buyer beware I guess.
Problem caused by mixing counting systems. Sales people use 1000 = 1K, computers use 1024 = 1K. Computers always find fewer K or GB on a card because they use binary to count, whereas humans and sales people use decimal.