Because the globe model is not accurate at all, south of the equator.

To illustrate this easiest, there was a group that was going to sail around Antarctica.
So, on a globe, you should easily make that journey in a month.
After a year, they gave up and turned north.

Every time they took a reading, they found that they weren't as far along as they thought.

So, if you try to sail around South Africa by going so many nautical miles, and then turning north you find that you haven't actually gone far enough, and you turn north and smack into land.

(it is not a "little" pear shaped.)