So it should return a big decimal with scale 5 and not depending of the value from my point of view
Where does it say that?
It simply says the value is returned with the full precision. So the value is not truncated or rounded in any way.
It does not claim that's the defined precision for the column.
My understanding is that the part "full precision" distinguishes this method from those (deprecated ones) that return the value with a given precision. ResultSet (Java Platform SE 6)