This has been covered in threads here if you search, but basically if players do more than 50% damage to an alien who is then killed by a turret, the team gets the stage point, otherwise the stage point is lost to the turret.
Sometimes as a dretch, it makes sense to hump a turret if you are shot to 15hp and doubt you can make it to safety, but when you do this, the human who shot you still gets the full 175 cash even though there is no stage point or kill credit given. Dretches that hump turrets after being shot to 10hp are feeding the stage advancement.
Same goes for humies, if you get a headbite but dont land the finishing blow, or even an unarmoured torso and foot bite, you can run away and still get the point when they step in acid or whatever.