Byte order marks.
I'm not sure how programs really determine the usage; the wiki entry indicates that utf8 doesn't require them and does not comment on if its recommended for or against their use. Presumably I would gather that the utf8 would be the standard, and just ansi ascii would be utf8 encoded so it appears per normal in a utf8 charset file. I love that the article explicitly mentions PHP and what bom will do to the http headers.
Lols what a pain.
Oh, best I know notepad doesn't have a non-bom utf8 option. Could be wrong though.