Ytdyklly

Is there any reason why some words are spelled with -zz-, but others are spelled with -z- between vowels?

My understanding is that either spelling can correspond to either of the following two consonant sounds: a long voiceless affricate [t.t͡s] or a long voiced affricate [d.d͡z]. (Phonologically, I've seen sources that indicate that Italian speakers may think of these as /t͡s.t͡s/ and /d͡z.d͡z/ respectively.) The linguistic sources I've read say that single [t͡s] and [d͡z] don't occur between vowels in (standard) Italian.

But some introductory descriptions that I see online use wishy-washy phrasing, like this one here:

Double z is pronounced almost the same as single z.

("Italian Language Guide". Bolding added by me.)

Is there any basis for saying that Italian speakers really pronounce -zz- "almost" the same as -z- rather than exactly the same as -z- (between vowels)?

If they are pronounced the same (as I think), why are some words spelled with -zz- and others with -z-?

This isn't a complete answer, but while Googling to try to find out more, I came across the following explanation for why some speakers might think there is a difference in pronunciation:

The orthographic distinction between 'z' and 'zz' induces some speakers to distinguish length in pronunciation (spazi 'spaces' spatsi vs. spazzi 'you sweep' spattsi) despite the fact that, in the standard language, both spellings are pronounced long in intervocalic position. Given that northern Italians often fail to observe consonantal length distinctions, in their speech the differentiation of the two 'z''s may arise from hypercorrection.

(Martin Maiden, 1994, A Linguistic History of Italian, 3.1.3.4)

Maiden goes on to say that

In some southern varieties, however, 'z' is pronounced long when it continues original consonant + [tj] (e.g. in concezione 'conception', azione 'action' < conceptionem, actionem), but short when it continues original vowel + [tj] (e.g. in nazione 'nation', stazione 'station' < nationem, stationem).

But the distinction for this group of speakers doesn't seem to be related to the spelling. So I still don't know where the -zz- vs. -z- spelling distinction comes from.