I reckon most hymns are set a bit high for the 'average' woman's singing voice, at least - on the unscientific basis that an untrained singer is most comfortable with a chest voice, and around a C' I for one have to go into a head voice, which is a bit of a strain (for this untrained alto-ish not-very-good singer, anyway!). Most hymns seem to hit highest notes that are C' or D' - so people transpose down to avoid those? At school we always had piano accompaniment and had to sing at pitch, and the singing was definitely quietest if the melody hit a D' or above.