What do "USP 26" and "USP 42" mean?
I was able to reach the manufacturer and ask her directly. USPXX is the &quot;edition&quot; of the US Pharmacopeia's &quot;exams&quot; that a product must pass in order to be considered pharmaceutical grade. As time goes on, these exams may get updated and improved. So a higher number USPXX means that a product passed a more recent (and likely, more demanding) USP set of exams.
Therefore, USP 26 is a cheaper PVP, that may not be able to pass the USP 42 standards. Or at least, wasn't tested on them.
On the other hand, USP 42 is a more expensive PVP,  that can and did pass the standard.
As for the actual differences between both standards, I don't know them. So I can't say if the difference between then it's significant or not. It seems USP guidelines are not available freely.
Edit: Todd Minehardt's comment had the right answer it seems.