GMS Monster Book (post chaos)

I think it has to do with your set. Your allstat is the regular stats, the set is the pot.
If your set has qualities that can only be gained from a certain rank of potential, then it's automatically that rank.

Expanding on this, the sets that give 15% PDR or % all stat are epic and the leafre and future ereve sets are unique( Leafre gives 30% PDR, +1 All skill, and future ereve gives 9% att/Matt, both of which are unique potentials.)