I came across something on tcp wiki that confuses me about its checksum

it says that for TCP_checksum_for_IPv4:

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 text.

I do not understand what that means, if something is a one's complement sum that means that it has inverted the bits. that what makes it a one's complement. Why then, is it taken through a one's complement again?

I guess I am not sure why the sentence says that instead of this

The checksum field is the rightmost 16 bit of the one's complement sum of all 16-bit words in the header and text.