> but oral language changes faster than written tradition

Yes and no! Sure, if they evolve at different speed, oral changes faster than written. But, in order to have any difference at all, you have to have accepted them to diverge at a certain moment in the past.

Luckily :-) this was never the case in Italy. Starting at least with Romans, written has always been meant as a phonetic transcription of the language. The pronunciation rules did change during time, but the underlying correspondence principle has never been given up.