Torah was dictated by G-d to Moses and is exact by letter.
Mishnah and Gemmorah (and all subsequent books) are man-made and written loosely (at least not letter-exact - see תנא ושייר and more).
The tradition says that the books of Na"ch were written by people with Ruach Hakodesh, not strictly dictated by G-d. [@Loewian suggested to differentiate between Prophets and Writings as they don't share the same level of ... something]
In what category of exactness do the books of Na"ch fall?