Completely anecdotal thoughts are about to follow:
I believe the aggro mechanics are different depending on which map you’re on, and possibly even down to the type of mob you’re facing. I think Anet has even played with these mechanics since the HoT launch, especially in portions of the raids.
It may that the aggro mechanics are more varied than vague, making it even harder to test, let alone define.
If thats true, thats awesome.