This is kinda "out there" but I had an original idea (or at least I thought it was original, until now) to make the purfling point outwards like the OP's example.
You may ask, why? Well, first ask yourself, what is purfling's function? To keep cracks from traveling any further past the purfling. Keeping the cracks more controlled prevents further damage to the remainder of the belly.
So, in this case, having the tip of the purfling (the point where the purfling meets in the corners) have a more perpendicular direction in relation to the grain protects more surface area of the corners as this purfling pierces through more lines of grain at the top compared to a corner that is angled upwards and more parallel to the grain like the blue line in this image.
Notice how the tip of the purfling starts to reach an asymptote almost at the nearest grain line. If you have the corner angled outward, (not pictured here in this graphic) you can intersect with more grain lines and thus, protect more of your corner from chipping off.
I think this idea supports why the 1680 Rugeri has this odd purfling characteristic only on the belly rather than the back, because the top wood is softer and more likely to lose its corners.
Just a theory, I do not have the skills or tools to test it out nor do I even know if what I said makes sense, just my two senseless cents.