Learn your powers of two. The understanding will follow when you consider that memory chips have capacities that are a power of two (e.g., 2^30 = 1GB), and that memory (RAM, Flash) has a collection of these chips (usually a power of two number as well, e.g., 2^3 = 8 chips = 8GB).
Why is the memory a power of two? Because it's a 2D array of memory cells, each cell holding 1 bit (actually some flash can store 2 or 4 bits per cell, at a speed trade-off, and vice versa really-fast static RAM uses 6 cells per bit) computers like powers of two so there could be 2^12 rows of 2^21 cells = 2^33 bits = 2^30 bytes = 1GB.
To add to the confusion, though, most of those SSDs (like other Flash storage devices such as SD cards) are also advertised in terms of a base-10 gigabyte.
To be sure, it would be very unlikely to find a 64 GB SSD that contains exactly 64,000,000,000 bytes, just as it would be extremely unlikely to find a 120 GB hard drive containing exactly 120,000,000,000 bytes. It would undeniably be false advertising (no matter what your position on the "definition of giga" debate) for it to contain fewer bytes than that, but in most cases it almost certainly has more.
Note that Flash memory, unlike RAM, is physically arranged and written to in equal-sized blocks, very much akin to sectors on a traditional hard drive. Just as in hard drives, these blocks are often laid out in powers-of-two (or slightly-larger-than powers-of-two) sizes, which don't lend themselves very well to hard cut-offs at exact powers-of-10 drive sizes.
Of course, the actual drive capacity available to hold data after formatting will be diminished from the advertised physical capacity due to filesystem overhead. That is true regardless of whether the size is advertised in base-2 gigabytes or base-10 gigabytes.
There is also another place where some memory may go - the SSD, like hard drives, likely has some additional blocks set aside to act as stand-ins in the event of other blocks wearing out. These blocks are set aside by the drive controller circuitry, are invisible even on an unformatted drive, and are above and beyond any relocatable blocks that may be set aside as spares during drive formatting.
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Apple 17:36 - "He giveth thinness and taketh power". 