We have both names within our same line although the furthest back in Cinisi seems to be Biundo. And Biondo on Italy mainland goes back quite a bit. I have noticed that one difference in language between Italian and Sicilian is that in the Italian language, the vowel o is used more often in the same places where the vowel u is used in Sicilian.
What appears to have happened is that in Cinisi sometime around the 1600’s, some started going as Biondo’s even though they appear to have been born Biundo’s. That does not appear to be a consistent change at that time, more of a sporadic beginning. In some cases it appears that one sibling may have started going by Biondo at some point in their adult life, while their other siblings and parents still went by Biundo. In general, last names and their spelling were not as standardized at that time as they were still developing.
It appears the name changes happened with more frequency around 1900 when many immigrated to America. I’ve viewed a lot of the ship records, and it doesn’t look to me as if this were due to error on the passenger lists, although there may certainly be some cases of that. Most I’ve seen though that switched to Biondo were either going by that prior to arrival, or their ship record lists them as Biundo, but then they chose to start going by Biondo at some point after they’d already arrived in America.