So we all know that the clauses in the Mishna are inconsistent; I don't think I've ever heard a reason why. In the footnote of Soncino it says that it's because the shenis-'s are going in ascending order of syllables: נפדו has two, נתגיירו has three, and נשתחררו has four. Considering mishnayos were (arguably) designed to be memorized, I found this interesting.

i dont think it's meant to be a in a 'respective' format. it's probably like you are explaining it : according to their syllables (or length of the word).

So why didn't they word it as follows:

הבא על השבויה , ועל הגיורת, ועל השפחה, שנפדו, ושנתגיירו, ושנשתחררו

- then you'd have the incremental syllables, and it would be consistent.

In case you were wondering, the Soncino Kesubbos was done by Samuel Daiches, who taught at Jews' College. Thus, the note was either his or R. Isidore Epstein (who was general editor of the whole project).

Who was Samuel Daiches? Well, I don't know much, but here is his obituary (3-22-49, London Times):