Doomspike... Think of it as symmetry. Take this example. You're the blue dot, the yellow is doomspike, the black line is the path of your Doomspike from you.

So let's say you're targetting the mob on the far side, directly opposite you. Your path is going through and directly hitting all mobs, except 2 of them. This would be considered most optimal use of Doomspike as it takes up the largest area of the circle where the mobs are concentrated.



But, let's say we're in the same place still and we're now targetting a mob that's not directly center. Look at the amount of mobs we're hitting now.



That's how Doomspike works. It's a 100% positional AOE and you need to always ensure you're perfectly opposite of the target you're attacking. The size of the AOE is always the same. Your target and your position to your target just plays eye tricks on you and makes it think it's smaller than it really is.