One reason, and Leaf and Donruss loved this, is where the same set has two different types. Release Notes normal explain, but I see they aren't loaded for this set. Using your example, let's say Statistical Standouts there were 1000 SN'd cards printed. The first 100 (1-100) may have been Die Cut (or Canvas, or AU or something else) and the other 900 (101-1000) may have been in a different format. Leaf didn't break up the SN numbering (stopping at 100 and retsart at 1) so they are one continuous set. This SN/PR annotation was a way of showing the that the card may show a SN up to a certain number, but only do may (the PR) actually exist in that format. That's what you'll see thing s like Statistical Standouts and then Statistical Standouts Die Cuts.
Again most sets like this has Release Notes that help explain, but not every set has them loaded yet.
Tired and trembling
I am descending,
will I have to stay here
and live this life again?