Apparently, somewhere in the distant past, somebody in authority miscounted what they had seen thru a microscope and the number of human chromosomes was deemed to be exactly 48. This nice, round, factorable, pleasant number was promulgated, promoted, and printed in myriad textbooks. I imagine that, for decades, some medical students were required to "see" all 48 thru their microscope oculars, at peril of flunking their laboratory exercises in karyotyping.
Sometime later, the mistake was caught and corrected, and by now all textbooks and most M.D.s have caught up with the new truth. So, now at last, all is right with the world -- at least until a new state is admitted to the Union.