Two reasons: 1.) The "actual available capacity" is likely referring to how it needs to store information about the filesystem on the device, so you can't use

Two reasons: 1.) The "actual available capacity" is likely referring to how it needs to store information about the filesystem on the device, so you can't use it to store, say, a 64 GiB file on it. 2.) There's been confusion as to whether a kilobyte, megabyte, etc is a multiple of 1000 or 1024. Because of this the same file will show a different size depending on whether you're using Windows or Mac. kilo- as a prefix denotes multiplication by a thousand, but tradition has made it necessary to introduce the kibibyte, mebibyte, etc.