I can confirm now - they have been installed the wrong way. One popped off this morning and I can see its the right dorsal vent installed on the left side. I'll use the hot water technique to see if I can remove the other one and swap them over to the correct sides.
Can't say for certain, but very likely just NECA making f**kups in the factory. I've had Alien Warriors with two right sets of dorsal tubes, upside down ones, etc.
I have a couple of Neca's awesome Kenner homage Gorilla Aliens. I have noticed that they have different dorsal vents - one has the dorsal vents pointing up, the other pointing down. Was this a manufacturer variant or an unintentional factory error? Which one is the regular version? Has anyone else had similar variations? Here are a couple of photos (I've currently repainting them in film colours)