the stars/asterisks mean that your performance is way above what they can tell you (or way below than what they can tell you if the bar is down instead of up). You'll notice sometimes that there is no bar at all, and that happens when the specific test form you did had only 1 or 2 questions on that topic (if any) so they can't reasonably tell you how you performed in that subject.
Notice how your total lack of bars typically happens with immunology. It's because you'll rarely see a good # of questions on pure immunology on any test form. So most of the time they just leave that bar out. OTOH, you will not probably see a total lack of bar in neurology because let's face it - it's a pretty high yield topic, and almost every test form will have a decent amount of neuro questions.