Question Why does my SD card inaccurately show available storage?

Jun 29, 2020
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I recently bought a SanDisk ExtremePro microSDXC UHS-1 card with 64gb storage memory for my phone (Realme 3). I haven't been using it for that long but it started saying that my SD card was full at around 500mb space left unused. So it's supposed to mean that I have taken up around 63gb of the SD card.
I checked multiple times, accounting even for hidden files but the amount that shows up is always just around 43gb. Apps do not count because Realme doesn't allow it's apps to install on external storage. Pls help me, I feel like the 20gb is such a waste.
That is almost 1/3 of storage space that is missing and unaccounted for!
Hopefully the solutions you can come up are mobile friendly because I have difficulty accessing a laptop/PC atm.