What is up with microsd cards having less than advertised capacity?
I have been shopping for a new card and looking at the screenshots and a lot of the cards are smaller than advertised.
I am not talking about the difference between GB and GiB. They literally have less. For example, I saw a 512GB card that had 500,313,358,336 bytes.
These are from major brands…
Contents may settle during transit. 
Perhaps it is due to this explanation, which suggests that during testing bad blocks are flagged, and effectively removed from the usable storage of the device: https://superuser.com/a/1848067. Probably manufacturers have to cull less inventory if they accept more defective pages per device.
It seems like dropping below the advertised capacity should be forbidden, but most likely there is some super vague disclaimer on the package like “actual usable capacity may vary.”
I’d wonder if it’s a variable of manufacturing, might multiple devices of identical model, all have varying bytes available?
I would see what fdisk is saying.
I suspect you are right. It is probably some type of yield issue. It seems like there should be some reasonable variance though. A capacity loss of nearly 3% seems really unacceptable to me.