And why is that?. Dan's Ghostly Wail is just like Danny's. It's far superior to his normal self. Literally everything about Dan is higher than Danny. Strength, Durability, Speed, All his abilites are far more developed. Danny just learned the Ghostly Wail before Dan because it was brought out through desperation while Dan trained and unlocked it.
Not only that but Dan is Danny. All of Danny's powers come from his ghost half. Dan is Danny plus Vlad's ghost half fused together with 10 years of growth. I don't really see why his Ghostly Wail would have become weaker than the Danny's when they are not only the same move but they are from the same person. One is just vastly stronger than the other.
However you example doesn't really fit the situation. 8-C to 5-B is far bigger gap than Low 6-B+ to Low 5-B. Not only that it doesn't take into account that it's a move that's already showen to be orders of magnitudes greater than his base and doesn't scale to his normal AP and doesn't scale to anyone else.
The only real issue here is Calc Stacking which I assumed but wasn't 100% on. DontTalkDT just confirmed it. Regardless, I'll close this given it's calc stacking (using the difference between Danny's calc and Vortex's calc to try and get Dan's Ghostly Wail AP) and I don't wish to argue about multipliers even if I think it's not an issue here, others do.