Why the capacity shown in system is different from the named capacity?

Why the  actual capacity is different from the named capacity?

It’s caused by the different calculating way of computer and SSD maker. And, in order to make better performance, we will preset OP room for some special applications, which will make the real capacity smaller than the name capacity. The OP room is for better use of Wear Leveling, Garbage Collection and Bad-Block Management.

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