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https://www.quora.com/Why-does-every-external-mass-storage-device-have-less-capacity-than-advertised
I need a laymans explanation
This spells it out about as simply as it can be. Otherwise, you’re welcome to peruse the 3.5 billion results from a Google search on the subject here and see if any of them explains it any better. 
There might be two possible reasons either you make some files hidden and forgot or nay vrus attack. Simply check for hidden files fist. Or scan the sd card using any antivirus. I also faced same situation in which my 64 GB sd card showing only 40 GB space. Then I scaned my sd cared and everything is fine for me.
A computer uses 1024 bytes for accessing the sd it is a 128gb but shows it in bytes if u check left on ur mouse on the usb drive it will show the properties at the very bottom of the list it’s a thing with computers that’s hard to explain.
