Alien blood/acid in the film

Rating: Null/10 (15 August 2004)

***Possible SPOILERS below***
Is it just me, or was the alien blood very selective about what it burned in the film. A little bit of face hugger blood in the first 'Alien' burnt through several decks. This new blood must be very diluted. ;)
The first time a Predator sliced an Alien with his claws, they melted... cool. But then the main Predator sliced through an Alien's head with his disc and nothing happened (in fact, the alien doesn't even bleed). Then Alexa impales one with her makeshift spear (I believe the tip is an alien tail, so no biggie there), and neither the staff nor her were ever burnt. It also looked like a bloodless kill. And wouldn't an alien head be full of blood and stuff (Alexa uses one as a shield). Did they sit there for a long time draining the thing? Oh that's right, Alien heads don't have blood at all (according to the disc scene).
It's almost as if they paid homage to the acid a few times, but essentially ignored it. Thoughts?