The rule you mention involves final sounds all of which contain either an 'S', a 'Z', a 'SH', a 'ZH', a 'DG' or a 'CH' after which it is obviously rather impossible to pronounce either 'S' or 'Z', so what happened was that the original ES was retained and not shortened as in all other cases.
A similar phenomenon applied to the alternation between T/D and ED in the past tenses of verbs: consider LOVED (LUVD), KISSED (KIST) as opposed to WAITED: it is simply impossible to pronounce WAITT, so here too the original ED was retained.
The plural of "quiz" is spelled with double "z" while the plural of "box" (and sometimes "bus") is spelled with single last consonant. Is this the general rule to double the last consonant to keep the syllable closed?
Citing Merriam-Webster "plural bus·es also bus·ses", this makes me think that "buses" is more common.
The "es" is added to such words for the sake of pronunciation.
Far better to say "ditches." Or take the word "whacks." The word "waxes" would have to be pronounced like "whacks" with an added S, or whacks-S, without the "es." So you see, it just works better with the "es," makes things easier to say.However, it's obviously a special case, and I've put forward one suggestion as to why that is; John Lawler has another which makes a lot of sense to me as well.I think the OP was asking for a general rule - but we all know that there are always exceptions. Code List: A list of all coded values for a particular field and the meanings they represent.A code list can be associated with multiple columns in a single data file or multiple data files.