Quote from the RFC791, the Internet Protocol definition:
"The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header."
And this is the only mentioning of the checksum algorithm in the whole RFC! How does someone come up with a code like this based on such description, i have no idea. I guess that everyone just copy-pastes code from an existing IP implementation without actually thinking what it was actually supposed to do.
I'm surprised that the today's Internet has its foundations in a definition as the above.