I think it has something to do with the focus of the person. Studies have shown that people tend to rate public speaking more fearful than death. It's something about the vulnerability of the situation and that it's all about you (the person testing, speaking, performing, etc.)
When I was in music school...I had no problem performing for my teacher or freinds. But when it came time for a public perofrmance or jury performance (graded) I'd have the worst panic attack...adrenaline dump...awful feeling. Anyway, I never conquered it in school..it wasn;t until later when I was playing parties and weddings that I got over the fear of public performance. Basically, I stopped thinking about me and started thinking about those whom I was playing for...it was their party/wedding after all. Same holds true of public performance...teh audience is there to be entertained...it's really not about me and whether or not I play as well as I would like. ANyway, still nervous now...but energized not paralyzed.
In a similar vein, I've developed an informal sub-specialty in crisis counseling dealign with suicide. When I'm with a client, while nervous (it's a terrifying situation), I'm able to let me training work for me..make that connection, establish a life-line...the fear energizes me. However, when I recently had to demonstrate my counsleing technique at a workshop in fornt of my peers in a mock situation...I got really scared and didn't feel that same energizing as I would in a real situation. Again, I was worried about what myu friends and colleagues would think about me. Whereas I'm normally too concerned with getting my client to safety that to worry abotu my ego too much.
It seems to me to be analogous to what doctors, lwyers and cops do. When they're working...they focus is on something or someone else...when you test...it's all about you. That's a ahrd place to be if you're not prepared.
Ok, enough rambling for me too...