First of all, I'd like to acknowledge that the poison mechanics are extremely complicated, and I'm not going to pretend I understand even half of the explanation in that link. That being said, I'm confused by how the game reports changes to the Assassin's Scorpion Blade skill/spell when investing skill points into it. I'm going to take some time to lay out exactly what is confusing, and then there are a couple simple questions at the end.
For the sake of this experiment, I took a level 51 Assassin, freshly respec'd, with no equipped gear, and base stats (no points assigned). I maxed her Way of the Spider skill at 25, and then put a single point into Scorpion Blade.
At this point, when hovering over Scorpion Blade in the skill tree, the game reports that the skill is at level 1, and it will do 646 damage every second. Since the skill tooltip only shows numbers to the nearest 10th of a second, I'll be calling that increment a tick (even though that's probably not the length of time the game's actual ticks are). So at level 1, the skill does 64.6 damage per tick.
The skill tooltip also tells you what about the skill will change when you invest another skill point to raise it to level 2. At level 1, it reports that if you increase the skill to level 2, it will now do 619 damage every 0.9 seconds, which works out to about 68.7 damage per tick, a minor but decent increase.
Here's where things get wonky. When I added another skill point to Scorpion Blade, the skill tooltip now reports that the skill is doing 646 damage every second - not the 619/0.9sec it had just told me it would be changing it to. In other words, nothing (apparently) changed except the level of the skill, and how much mana it takes to cast it.
At level 2, the tooltip says that increasing the skill to level 3 will make the skill do 619 damage every 0.9 seconds - exactly what it said it was going to do this level. When you increase it to level 3, nothing changes - it stays at 646/1sec, and says that level 4 will give you 619/0.9sec.
This stays the same until level 5. At level 5, it's still doing 646/1sec, but now it says that increasing the skill to 6 will change the damage to 593/0.9, or about 65.8 damage per tick. A tiny bit more than it was doing at level 1, but still not the 68.7/tick it said it was going to be doing all the way back at level 2. But when you actually raise the skill to 6? Now it does 619/0.9sec.
At level 6, it does 619/0.9sec, and it says level 7 will give 593/0.9sec, but it stays at 619/0.9sec.
At level 7, it does 619/0.9sec, and it says level 8 will give 567/0.8sec, about 70.8 damage per tick, but it actually changes to 593/0.9sec.
At level 8, it does 593/0.9sec, and it says level 9 will give 567/0.8sec, but it stays at 593/0.9sec.
At level 9, it does 593/0.9sec, and it says level 10 will give 542/0.8sec, or about 67.7 damage per tick, but it actually changes to 567/0.8sec.
At level 10, it does 567/0.8sec, and it says level 11 will give 542/0.8sec, and it - oh, this time it actually does change to that.
Skipping to the end, at level 25, it's doing 516/0.8sec, or about 64.5 damage per tick, which is a teensy bit less than the damage at level 1. Granted, it throws more knives, and enemies stay poisoned for longer, so I wouldn't be surprised if the skill does end up more powerful overall... but that's not my point. My point is the tooltips are WONKY.
So my questions are: Is anything actually changing between individual skill levels, where it says the damage will change, and then it says it hasn't? Is this intended, and if so, why? If not, is it a known error, or is it some unfixable side effect of the engine?