I've known singers who would be classed as tone deaf from singing "on stage" but get them at the back of the room in chorus and you wonder where the quality suddenly appears from. The real explanation in these circumatances is nerves, adrenalin causes muscles to twitch, the fight or flight syndrome. The larynx IS a muscle. Of course having someone else to pitch for you is also a remedy in these cases.
Misdiagnosis would skew the tone deafness figures.