The criteria for a "Normal" lens is an arbitray value (equal to the diagonal of the format) chosen for the lens designers more than to duplicate the perception of vision.

Somewhere, early in my photography, I read a comprehensive article about a study of human perception, and the conclusion was that human visual perception was equivalent to a 90mm - 100mm lens in 35mm format. The classical choice for that focal length in portraiture is easily understood. That would be equal to - what - something like 150mm in 6cm x 6 cm.