Memory organisation of MIFARE 1K
You're reading (and doing your sums) backwards.

You have 1024 bytes of data space.

That data space is split into 16 sectors, so 1024 / 16 = 64 bytes per sector.

Each sector has 16 bytes reserved for keys etc, so each sector has 64 - 16 = 48 bytes of user data space.

48 bytes of user data space per sector multiplied by 16 sectors is 768 bytes.

Incidentally 1024 - 768 = 256, which is what you were calculating before.  What you calculated was the amount of the 1024 bytes that is reserved in total for the keys and access controls.