I haven't looked at the PLB premix, but I don't see any reason for the order of exercises aside from variety. It is usual to have weight workouts go from largest muscle groups to smallest (lower body, back, chest, shoulders, triceps, biceps...sometimes with back/chest and triceps/biceps reversed).
The PLB premix DOES have a certain logic, in that it goes from a larger muscle group that uses a smaller muscle group as an agonistic (assisting) muscle (ie: biceps are worked when doing rowing exercises, as are triceps when doing chest presses), but it's a combo I'd personally use as a split (back/bicep day 1, chest/tricep day 2, and shoulders day 3).