There are quite a few differences that "aren't", meaning there are more than five on each level - annoying to have obvious differences not registering.
Half the levels complete themselves, the rest are so simple that it's really like clicking 'Continue' rather than taking a shot (and you really do only need one shot to complete the levels in which you're required to do something)
lvl5 - the ammo goes off the screen as the arm flies up and disappears, which somehow counts as a shot despite nothing being fired. I can't go any further in the game if I can't fire anything on lvl5...