Back in the days of studying music theory and writing choral music, I was taught the range of voices much as are outlined on that website. I was told that these are general ranges and if the music seemed to call for it, one could write for a voice to hit a note or two outside of its designated range. Since it was a class, we were rather timid about "coloring outside of the lines". But certainly there have been composers who have done just that. They have the altos singing soprano notes - hell, they even have tenors singing soprano notes!

My choir directors always encouraged us to try to hit the higher or lower notes as written but most said it was usually okay to drop down a third or go up a third if we just couldn't reach the written one.

There are always people who are blessed with being able to sing "outside the lines". The late Minnie Ripperton could bounce around in three octaves and never strain a note. Her falsetto was so clear that you often couldn't hear the change of voice.

It has always seemed to me that more women than men were blessed with such broad ranges. Most men I have known were hard pressed to sing the whole generally accepted range of both bass and baritone or baritone and tenor. And it is a rare tenor indeed, who can sing above an A without sliding into a falsetto. But I used to sing with a "first" soprano who could sing anywhere down to all but the lowest bass notes. Her falsetto wasn't half bad either. I have known many other women who came fairly close to the same broadness of range. - Pretty impressive!