I saw your post from years ago--I too have come across this but haven't found a definitive answer. I just assumed that Mary gave birth to a child that died in infancy, or at least sometime between the timing of the censuses. I can account for 4 of the known 13 to have been deceased at the time of the 1900 census--I would guess the 5th deceased child is this unknown baby.
As for the Mary vs. Martha issue--I really don't know on that one.