Because if you do it that way then the IDC refresh is staggered and the stagger ends up cascading and resulting in slightly shaky timing.
HP IDC TTT ?
If you use TTT again then the HP goes down.
If you use HP again then:
HP IDC TTT
HP ???
If you use IDC, then that clips heavily. If you use TTT:
HP IDC TTT
HP TTT IDC
Then the second IDC clips a bit (est. 2 ticks), but it is not horrendous. The problem is the next sequence:
HP IDC TTT
HP TTT IDC (note: the clipping here is already inefficient compared to the original H-IDC-P approach)
HP TTT ???
In this case, what do you use? If you use IDC, then it will again clip very heavily (est. 4 ticks). If you use TTT, then you will lose a GCD of DE uptime and end up with some CT downtime.
However, due to the 2.1 clip changes and the raw strength of CT, the clipping and downtime in this sequence is not "horrible". It's just less efficient than:
H IDC P TTT H TTT
P IDC H TTT P TTT
The HP IDC offset approach is actually less than a percent weaker than the alternating H/P approach, so use whatever you're comfortable with. They're both 24 attacks long.
