Hi Zyozu,
So, the 9 and 13 frame marks you are seeing for WW/DD and HotA are because you still have the Echoing Fury slider at max (5) stacks. In a single-target boss fight, you will not be able to keep those stacks, and of course in a boss fight with adds, those adds will eat a lot of Stricken, so the actual rate of stacks applied to the boss will be much lower.
The 7.3 frames per stack for the combo of WW and DD comes from my own test, which you can see here: Stacking Stricken with WW - YouTube. Basically, 493 stacks in 60 seconds, which works out to 7.3 frames per stack.
As for WHY it’s so fast: it’s because both base WW and the DDs can each stack Stricken independently. In addition… well, you’re probably aware that the ICD for Stricken is 0.9 * fpa (which is the frames per attack/animation of the proccing skill). Usually, the actual rate of stacks accumulated is just the actual attack rate of the skill in question, since that skill can “only attack as fast as it attacks”, no matter how low the ICD might be. But in this case, WW can hit on the frames after the DD icd elapses, and DD can hit on the frames after the WW icd elapses.
So, the actual rate for WW is 19 frames, and for DD, 14 frames. But, both of those can get stacks, and the icds are 17 and 12 frames, respectively. So, in 60 seconds, you could theoretically get 3600 / 17 = 212 stacks from WW and 3600 / 12 = 300 stacks from DDs, for 512 stacks total. Now, those WW and DD hits aren’t always going to fall perfectly, right after the icd elapses, which is why I got “only” 493 in my test. If you just used the base rate of WW and DD (19/14 frames), you’d end up with 447 stacks. So I believe the result will always fall between 447 and 512 stacks per minute, which is a stack between 7.03 and 8.05 frames.
Not many skills and effects operate this way. One of the only other places I’ve seen this sort of thing is with the Frenzy “chain” created by Bastion’s Revered, which can cause a higher overall stacking rate of Frenzy than its actual breakpoint- though only when applied to multiple targets.
I hope this is helpful!