I don't know if this is a bug but in the FAQ thread it says;
Egg - 0 posts - 1 Star
Facehugger - 5 Posts - 1 Star
Chestburster - 50 Posts - 2 Stars
Drone - 100 posts - 3 Stars
Predalien - 250 Posts - 4 Stars
Praetorian - 500 Posts - 5 Stars
Ravager - 1000 Posts - 6 Starsbut I've only ever seen Ravagers with 5 stars