Interesting that the PDs are referred to as "sentences." In the Saint-Andre translation I quickly counted only 5 as having more than one sentence. This may help to explain, at least to a degree, how they were divided into the current 40 PDs: the original groupings may have simply been separated into sentences.
Somewhere I read, although I can't remember where, that the authorship of the PDs was in question. Wherever I read it, I think a footnote connected the idea to an Italian source. Tantalizing, but not much help!