There are basically two reasons why a singer can’t duplicate the pitch that he hears?
Firstly, the singer has an “ear” problem. The singer cannot duplicate the sound he is hearing because he does not know if he is on pitch. He has not (typically) practised this from a young age and has developed no point of reference. The singer has not imitated sounds requiring the vocal cords and pharynx to adjust to different pitches. The singer cannot associate the pitch he hears with the coordination that his vocal cords need to make to duplicate the sound. This problem can be fixed with “ear training” exercises, and methods such as Kodaly and solfege.
Secondly, a “technical” problem. Usually this is evident with singers who sing louder as they ascend in pitch .The singer knows it is the wrong pitch but doesn’t have the control to fix it. This is especially evident in singers with the inability to decrease volume at a high pitch. This problem can be decreased with the help of a good singing teacher.