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.
