The difference in pens is because of the different ways a right or left handed person writes. When writing from left to right across a page, a right handed person pulls their pen across the paper with the right side of the nib making contact with the paper, while a left handed person pushes it with the left side making contact.
In biros and other roller-ball-type pens this doesn't make any difference because the roller ball can move in any direction to spread ink. It makes a difference to fountain pens because the point has to be designed differently depending which side of the nib will be pushed or pulled across the paper to make sure the ink gets to the paper at all and with any degree of consistency. Theoretically a leftie can write with a right-handed fountain pen, and vice versa, but it's difficult and the ink splashes and doesn't flow evenly because they're using the 'wrong' side of the nib, so it's very awkward to write with at all, let alone neatly.
Ah, the mine of useless knowlege one acquires by being brought up by two teachers...