they are all spelling errors made by individual posters or menu makers, not secondary spelling versions. Visit the Parma, Italia and you'll see how it's spelled. Visit the companies' websites that produce the product. Using common spelling errors from posters on a website or from menus as proof of your foible, is every bit as bad as using wikipedia as a reference source.
Truth is, if "prociutto" was a correct spelling, it wouldn't be called "prô-'shü-(,)tô," since the "sc" is what gives the "sh" sound like the word "crescendo." The fact that some chef in Nantucket doesn't know that, doesn't mean squat. It's wrong.
In fact, if you google "prociutto" in Parma, Italy, you get ZERO hits for that word. ALL hits spell it "prosciutto." Apparently, in Italy, they don't misspell it.
It's ok to be wrong. It's not the end of the world. Similar to your, hopefully, revelation, I mispronounced "fustrated" for years, until one of my friends said, "dude, it's 'frustrated.'" And I said, "that's what I said...isn't it?" And he said, "no. you said fustrated." And I said, "why didn't you tell me that years ago?" He said, "I never found the right time."
That was "fustrating" because I hadn't heard of a silent "r" either.