Because for Unbloodeds, some armor/weapons are acid proof and some aren't?
I'd just admit it. As much as I love AVP, it's obvious Anderson overlooked those things.
Of course you could say that Anderson made them that way on purpose because he favored the Aliens, but that wouldn't be true now would it? It is not true because Scar took out SEVERAL Aliens. Alien fan's worst nightmare right?
Every Alien since the first movie (with the exception of the third) has been treated like cannon fodder, like a bug. Whether this means making them an unstoppable horde or extensive target practice, they appear in both of these ways for AVP. Which makes sense, considering that Scar gets his plasmacaster later on in the film, shifting the balance.
They might have trained for decades, but with Xenomorphs you just don't know.
They are their most deadly prey.
That's the best way to describe them: unpredicable. It's what makes taking down an Alien such a big deal. The point isn't to analyze them to the point where they become pushovers; it's to live off the thrill of the hunt, to prove that you can adapt to any situation and use the environment in your favor. Predators don't train themselves in handling a specific breed of prey, but rather in being an effective hunter on any world.