I think Kiya is on to something. Generally, I just let people know that I "startle really easily," "am jumpy," or "get surprised easily." Most people adjust their behavior accordingly--it's polite. On ocassion, I have to politely ask someone, "Please let me know you're coming in" or "Please don't surprise me." I still startle when people knock on my door or make deliberate footfalls, but it's not as bad as a total shock.

I don't think people even guess the reason--this is just how I am, end of story and curiosity.