Maybe because Noctis's u/f+3 is faster, like hopkick fast?
Majority of d/f+1 moves are 13 frames, but you can see how fast it is by trying to use that move to punish a move that is vulnerable by its number of frames. If Noctis's d/f+1 can punish a -13 move but cannot punish a -12 move, then...