Pitch is so very dependant on educational experiences as a child. Modern singers have not studied piano, eurhythmics, violin, prior to professional singing. Singing works on imprinting, layer after layer of experiences.
Many times, one cannot hear on the stage, or one's pitch is coming from somewhere far away from where one is standing. I always tell two stories, the first time, I sang a contemporary piece with orchestra, my pitch came to me from two miles away. The first time, I sang the Trumpet shall sound, I thought I was hit in the head by the trumpet.
I always wondered what was up with Kate Smith on early television. She couldn't hear the piano. She developed a huge set of hand signals. Spread up, slow down, etc.
I think a generation earlier, who had educational experiences, taken piano, rarely sang flat, in comparison to this modern era.