I’ve noticed this on a couple characters, but it’s especially bad on Alumon - his basic Actions seem to only do about 75% of the listed damage. Shadow Whip was supposed to hit for 299, only hits for 227. Soul Shard said it should hit for 486, only hits for 375.
Some other abilities seem to be affected too - Devastation was listed as about 1300, and with 2 stacks of Arcane Sickness on the monster it only hit for about 800. It’s hard to plan my turns out when the attacks often don’t hit as hard as they state they should be.
First off, you’re discounting enemy defense, which reduces the damage your attacks deal and isn’t factored into the tooltips (because how do the tooltips know which enemy you’re going to attack when your choose your target after choosing the attack?).
A second potential effect is that the enemy is higher level than you and I’m pretty certain that there are level difference modifiers: when you attack an enemy that is higher level than you, you’ll deal less damage than you otherwise would (going by the use of Piercing Attacks, which ignore defense; perked Rage Blade is an AoE Piercing attack; I’ve regularly recall seeing the attack do more damage to lower level enemies of the exact same type, but I might be misremembering). It’s not as huge as many other games do, but, stacked up with the defense difference, it can compound to feel extremely potent.
By default, I’d assumed enemies have 0 defense and Piercing was for enemies that use +defense buffs. Since he always seems to have the same penalty regardless of target, I suspect they do not have defense stats baseline.
This would explain it, and if it’s the case I wish it were called out somewhere. It’s already difficult getting through legendary when enemies do 1/3 of your life per hit and have a lot more hit points than your team, doing less damage on top of that just makes combat frustrating. Thinking you have time to heal because your damage dealer will kill the enemy only for them to hit for 75% of their stated damage and thus dying before the heal hits is no bueno.
That’s not true in the slightest. You can test this pretty easily with the Rage Blade perk (you can also test it with Knolan, who can make certain normal attacks Piercing), which allows you to take the same attack and turn it from normal Magic damage to Piercing Magic damage. Against the same level and type of enemy, with the same base damage listed on the tooltip (and the same amount of Overcharge consumed), same buffs, etc., you’ll end up dealing roughly 30% more damage with Piercing damage than you will with non-Piercing damage.
I tested this against Ancient Liches (and a few others in the Deep Snows, but the Ancient Liches were the first time I could control all of the conditions). The base damage was listed as ~5500 (even with all the same buffs and starting conditions, it alternated between 5541 and 5428; the end damage always listed the same however); without the perk, the damage was ~10500 (10481 to be specific) and, with the perk, the damage was ~13500 (13528), which is ~30% more damage due entirely to it being Piercing damage.
I learned, very quickly, that you can’t trust the tooltips to correspond perfectly with the results. The tooltips are an approximation that doesn’t take into consideration a vast majority of what’s going on. In all honesty, the only thing that they’re really useful for is giving you a number for comparison with your other options at that time.
In general, if you want to plan on killing an enemy, you need to have pretty significant overkill for it.
By that same token, once you’re reasonably optimized, it’s pretty easy to overkill stuff just by sneezing at it, even on Mythic difficulty (Mythic is a NG+ difficulty that is above Legendary).