Stalker,
Undefined,
17 years ago
It was really done just for "cool" effect. There are plenty of unexplained continuity errors such as this. Eg. in the first 2 Predator movies, the wristblades would become visible when extended, but were cloaked in the AVP films. I'm pretty sure even Anderson pointed out that he overlooked continuity for the "cool factor" in the DVD commentary.