I'm a retired newspaper copy editor. I don't know what the procedure was in 1908 at the Boston Globe, but I can tell you that when I was working, this sort of information was very often provided by the family, very often on a fill-in-the-blanks form provided by the newspaper. It was then typed in by newsroom clerks. If the form was filled in by hand (more likely than not, especially in 1908), it's possible that the clerk misread the submitter's handwriting. It's equally possible that the submitter ticked the wrong box indicating the best man's relationship to the groom. The bottom line is this: It's best to consider any information you get from a newspaper birth, engagement, marriage or death announcement as nothing more than a clue until you have confirmed the information from primary sources. In this case, that would mean finding out who Frank's parents were and seeing how they're related to (or if they are the same as) the groom's.
This, of course, is not to say you shouldn't trust anything you find in a newspaper. But you have to consider how the information was gathered. A news story, written by the reporter who gathered the information and edited by one or more editors, generally will be more reliable than a society-page announcement.
Deborah Blankenberg (JewishGen ID #613395)
Researching BLOCH/BLOCK (Germany to New York, Colombia and Missouri), BLINDER (Kishinev to New York via Poland? and Paris), KUSHER/KUSZER (Lodz vicinity to New York via Paris), GOLDSCHMIDT (Germany)