I think the Matriarch is somewhat nerfed because of situational considerations, namely the fact that you can destroy the web beneath her feet, causing her to fall and take substantial damage.
This effectively makes her hit point total *seem* higher than it is for practical purposes.
Now, in fairness, some people might not like the idea of "gimmicky" fights, but I think it's basically baked into the challenge rating of the encounter by design.
Yeah, speaking of that gimmick, I also don't approve. Even IF a spider took fall damage, it shouldn't be substantial. It's what? 30-50 feet? She's a giant spider. Real spiders can fall huge distances relative to their size and still scramble away as if unharmed. I just have a hard time with that gimmick which wouldn't be necessary with more appropriate stats and combat tactics.
I just wanted to point out that the challenge rating isn't as off as it seems because that gimmick lowers the difficulty.
In regards to not liking the gimmick itself, that's a fair argument. I don't disagree, not necessarily. (I'm not sure the same fall damage holds true when you change a creatures size, though. For instance, if you make an ant bigger, it doesn't keep being able to lift 5,000 times its body weight. Things like that change with scale.)
Anyway. I think from Larian's point of view, they figured it would be fun to let the player break the web and drop the spider, making the fight more dynamic by allowing interactivity with the environment.
