Dual Blade's "Attack Speed"

Hello, I am aim, Japanese JMS Player. Nice to meet you.

I'm making "Damage Per Minute Spread Sheet (MS Excel)" in JMS.
MapleDPM : http://kaedekouyou.blog97.fc2.com/blog-entry-781.html

This is almost Collect. But Some are not. Therefore, I want to ask your help.
This time, I want to know about "Attack Speed"


I know almost about "Attack Speed Calculation formulate." But some skills does not fit.
I want to understand why does it happen.


I use this formula

=CEILING ( Skill Delay *(10 +Weapon's Speed) /16 , 30)


Almost all skill's are fit with the formula.

But Dual Blade's skill : "Phantom Blow(720ms)" Doesn't fit in AS2.
On the other hand, "Blade Fury(720ms)" fit.

Formula / Phantom Blow(survey) / Blade Fury(survey)
AS2: 540ms /570ms /540ms
AS3: 600ms /600ms /600ms
AS4: 630ms /630ms /630ms
AS5 :690ms /690ms /690ms


Why does it happen??
My formula is wrong, or Dual Blade has some different Rules?