Why is the apostrophe positioned differently in "ones' complement" than "two's complement"? [duplicate]

Donald Knuth, that doyen of computer science, says in Art of Computer Programming, Vol 2.:

Detail-oriented readers and copy-editors should notice the position of the apostrophe in terms like "two's complement" and "ones' complement": a two's complement number is complemented with respect to a single power of 2, whereas a ones' complement number is complemented with respect to a long sequence of 1s. Indeed, there is also a twos' complement notation, which has radix 3 and complementation with respect to (2...22) (base 3).

APOSTROPHE RULES

From: yourdictionary.com "Apostrophe Rules - English Grammar Rules & Usage"

URL: https://grammar.yourdictionary.com/punctuation/apostrophe-rules.html

My edits indicated with [brackets].

[SINGULAR FORMS:]

Use an apostrophe + "s" ('s) to show that one person/thing owns or is a member of something.

[•Two's complement]

•Amy's ballet class

•Robert's car

•Ross's room

•Ross's sports teams

Even if the name ends in "s," it's still correct to add another "'s" to create the possessive form. It is also acceptable to add only an apostrophe to the end of singular nouns that end in "s" to make them possessive. In this case, you can show possession for Ross either way:

•Ross'

•Ross's

It makes no difference whether the item owned is singular or plural. We use "Ross's" to say that the room (singular) is his and that the sports teams (plural) are his.

[PLURAL FORMS:]

Use an apostrophe after the "s" at the end of a plural noun to show possession.

[•Ones' complement]

•The parents' bedroom

•The Smiths' lives

It is not necessary to add another "s" to the end of a possessive plural noun.

If a plural noun does not end in "s," add an apostrophe + "s" to create the possessive form.

•The children's rooms

[SUMMARY:]

Remember, a possessive noun [should have] an apostrophe and an "s" at the end. [Alternatively,] if there's already an "s" there, you can just add the apostrophe. If there's no "s," you have to add both - first the apostrophe, and then the "s."