SD cards can be shown to have let's say 32GB, but infact have 1GB because those memory cards have bad sectors.
What happens is you can still continue writing into the card but the old data get pushed into the bad sectors and are lost forever. Meaning you can lose something precious
I think companies QC their products and the rejects end-up in a pile to be recycled, and employees sneak some of the defectives into their pockets, take them home and profit majorly on the internet.
This is now very common, I am doing my test as I speak, because I think I've been scammed. If you voice your problems, at least someone can have a look at it and the sellers can own up to their responsibility.