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#11
Most SSD's have a certain percentage 5 to 10% of their capacity blocked (OP). This empty space is used by the internal system of the SSD to facilitate their internal cleaning process. Unfortunately they sell using the total capacity rather than the usable capacity. I suspect this is what you are seeing.
Some more math, following on what xtcrefugee and Snakeyes said:
The manufacturer's "64 GB" equates in Windows GB units, to 64,000,000,000 / 1,024 / 1,024 / 1,024 = 59.6 GB
You'll have a minor amount taken up in hidden partitions (more if there is a factory restore partition, say 5 - 20 GB). The 59.6 - 57.8 = 1.8 other GB is likely over provisioning allowance on your SSD, which would be about 3% in this case minus the small hidden partition amounts.
You may get a better accounting of your 59.6 "true" GB from the Disk Management console display.